tms 
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GIFT  OF 

ASSOCIATED  ELECTRICAL   AND 
MECHANICAL   ENGINEERS 


MECHANICS  DEPARTMENT 


Engineering 
Library 


UNIV    CALIF 

A  E  &  M  E 

ROOM 


UN  IV     CALIF 

A  E  &  M  E  l- 


GENERAL  LECTURES 

ON 

ELECTRICAL  ENGINEERING 

BY 

CHARLES  PROTEUS  STEINMETZ,  A.  M.,  Ph.  D. 

Consulting  Engineer  of  the  General  Electric  Company, 

Professor  of  Electrical  Engineering  in  Union  University, 

Past  President,  A.  I.  E.  E. 

Author  of 

"Alternating  Current  Phenonema," 

"Elements  of  Electrical  Engineering," 

"Theory  and  Calculation  of  Transient  Electric  Phenonema 

and  Oscillations." 


Edited  by 

JOSEPH  Le  ROY  HAYDEN 


Robson  &  Adee,  Publishers 
Schenectady,  N.  Y. 


Engineering 
Library 


Copyright  1908  by 


Contents 


First  Lecture — General  Review 7 

Second  Lecture — General  Distribution 21 

Third  Lecture — Light  and  Power  Distribution 35 

Fourth  Lecture — Load  Factor  and  Cost  of  Power. ...  49 

Fifth  Lecture — Long  Distance  Transmission 61 

Sixth  Lecture — Higher  Harmonics  of  the  Generator 

Wave 77 

Seventh   Lecture — High   Frequency   Oscillations   and 

Surges 89 

Eighth  Lecture — Generation 99 

Ninth  Lecture — Hunting  of  Synchronous  Machines. .  113 

Tenth  Lecture — Regulation  and  Control 125 

Eleventh  Lecture — Lightning  Protection 135 

Twelfth  Lecture — Electric  Railway 147 

Thirteenth  Lecture — Electric    Railway    Motor    Char- 
acteristics    163 

Fourteenth  Lecture — Alternating     Current     Railway 

Motors 175 

Fifteenth  Lecture — Electrochemistry 197 

Sixteenth  Lecture — The  Incandescent  Lamp 207 

Seventeenth  Lecture — Arc  Lighting 215 

Appendix  I. — Light  and  Illumination 229 

Appendix  II. — Lightning  and  Lightning  Protection. .  259 


789562 


Preface 


HE  following  lectures  on  Electrical  Engineering  are 
general  in  their  nature,  dealing  with  the  problems  of 
generation,  control,  transmission,  distribution  and 
utilization  of  electric  energy;  that  is,  with  the  operation  of 
electric  systems  and  apparatus  under  normal  and  abnormal 
conditions,  and  with  the  design  of  such  systems ;  but  the  design 
of  apparatus  is  discussed  only  so  far  as  it  is  necessary  to  under- 
stand their  operation,  and  so  judge  of  their  proper  field  of 
application. 

Due  to  the  nature  of  the  subject,  and  the  limitations  of 
time  and  space,  the  treatment  had  to  be  essentially  descriptive, 
and  not  mathematical.  That  is,  it  comprises  a  discussion  of 
the  different  methods  of  application  of  electric  energy,  the 
means  and  apparatus  available,  the  different  methods  of  carry- 
ing out  the  purpose,  and  the  relative  advantages  and  disadvant- 
ages of  the  different  methods  and  apparatus,  which  determine 
their  choice. 

It  must  be  realized,  however,  that  such  a  discussion  can  be 
general  only,  and  that  there  are,  and  always  will  be,  cases  in 
which,  in  meeting  special  conditions,,  conclusions  regarding 
systems  and  apparatus  may  be  reached,  differing  from  those 
which  good  judgment  would  dictate  under  general  and  average 
conditions.  Thus,  for  instance,  while  certain  transformer  con- 
nections are  unsafe  and  should  in  general  be  avoided,  in  special 
cases  it  may  be  found  that  the  danger  incidental  to  their  use  is 
so  remote  as  to  be  overbalanced  by  some  advantages  which 
they  may  offer  in  the  special  case,  and  their  use  would  thus  be 


PREFACE 

justified  in  this  case.  That  is,  in  the  application  of  general  con- 
clusions to  special  cases,  judgment  must  be  exerted  to  deter- 
mine, whether,  and  how  far,  they  may  have  to  be  modified. 
Some  such  considerations  are  indicated  in  the  lectures,  others 
must  be  left  to  fthe  judgment  of  the  engineer. 

The  lectures  have  been  collected  and  carefully  edited  by 
my  assistant,  Mr.  J.  L.  R.  Hayden,  and  great  thanks  are  due 
to  the  publishers,  Messrs.  Robson  &  Adee,  for  the  very  credit- 
able and  satisfactory  form  in  which  they  have  produced  the 
book. 

CHARLES  P.  STEINMETZ. 


Schenectady,  N.  Y.,  Sept.  5,  1908. 


FIRST  LECTURE 


GENERAL  REVIEW 

N  ITS  economical  application,  electric  power  passes 
through  the  successive  steps :  generation,  transmission, 
conversion,  distribution  and  utilization.  The  require- 
ments regarding  the  character  of  the  electric  power  imposed 
by  the  successive  steps,  are  generally  different,  frequently 
contradictory,  and  the  design  of  an  electric  system  is  therefore  a 
compromise.  For  instance,  electric  power  can  for  most  pur- 
poses be  used  only  at  low  voltage,  no  to  600  volts,  while 
economical  transmission  requires  the  use  of  as  high  voltage 
as  possible.  For  many  purposes,  as  electrolytic  work,  direct 
current  is  necessary;  for  others,  as  railroading,  preferable; 
while  for  transmission,  alternating  current  is  preferable, 
due  to  the  great  difficulty  of  generating  and  converting  high 
voltage  direct  current.  In  the  design  of  any  of  the  steps 
through  which  electric  power  passes,  the  requirements  of  all 
the  other  steps  so  must  be  taken  into  consideration.  Of  the 
greatest  importance  in  this  respect  is  the  use  to  which  electric 
power  is  put,  since  it  is  the  ultimate  purpose  for  which  it  is 
generated  and  transmitted ;  next  in  importance  is  the  transmis- 
sion, as  the  long  distance  transmission  line  usually  is  the  most 
expensive  part  of  the  system,  and  in  the  transmission  the 
limitation  is  more  severe  than  in  any  other  step  through  which 
the  electric  power  passes. 

The  main  uses  of  electric  power  are : 

General  Distribution  for  Lighting  and  Poiver.  The 
relative  proportion  between  power  use  and  lighting  may  vary 
from  the  distribution  system  of  many  small  cities,  in  which 


*!<*:  GENERAL  LECTURES 

practically  all  the  current  is  used  for  lighting,  to  a  power 
distribution  for  mills  and  factories,  with  only  a  moderate 
lighting  load  in  the  evening. 

The  electric  railway. 

Electrochemistry. 

For  convenience,  the  subject  will  be  discussed  under  the 
subdivisions: 

1.  General  distribution  for  lighting  and  power. 

2.  Long  distance  transmission. 

3.  Generation. 

4.  Control  and  protection. 

5.  Electric  railway. 

6.  Electrochemistry. 

7.  Lighting. 

CHARACTER  OF  ELECTRIC  POWER. 

Electric  power  is  used  as — 

a.  Alternating  current  and  direct  current. 

b.  Constant  potential  and  constant  current. 

c.  High  voltage  and  low  voltage. 

a.  Alternating  current  is  used  for  transmission,  and 
for  general  distribution  with  the  exception  of  the  centers  of 
large  cities;  direct  current  is  usually  applied  for  railroading. 
For  power  distribution,  both  forms  of  current  are  used ;  in 
electrochemistry,  direct  current  must  be  used  for  electrolytic 
work,  while  for  electric  furnace  work  alternating  current  is 
preferable. 

The  two  standard  frequencies  of  alternating  current  are 
60  cycles  and  25  cycles.  The  former  is  used  for  general  distri- 
bution for  lighting  and  power,  the  latter  for  conversion  to 
direct  current,  for  alternating  current  railways,  and  for  large 
powers. 


GENERAL  REVIEW  n 

In  England  and  on  the  continent,  50  cycles  is  standard 
frequency.  This  frequency  still  survives  in  this  country  in 
Southern  California,  where  it  was  introduced  before  60  cycles 
was  standard. 

The  frequencies  of  125  to  140  cycles,  which  were  standard 
in  the  very  early  days,  20  years  ago,  have  disappeared. 

The  frequency  of  40  cycles,  which  once  was  introduced 
as  compromise  between  60  and  25  cycles  is  rapidly  disappear- 
ing, as  it  is  somewhat  low  for  general  distribution,  and 
higher  than  desirable  for  conversion  to  direct  current.  It  was 
largely  used  also  for  power  distribution  in  mills  and  factories 
as  the  lowest  frequency  at  which  arc  and  incandescent  light- 
ing is  still  feasible;  for  the  reason  that  40  cycle  generators 
driven  by  slow  speed  reciprocating  engines  are  more  easily 
operated  in  parallel,  due  to  the  lower  number  of  poles.  With 
the  development  of  the  steam  turbine  as  high  speed  prime 
mover,  the  conditions  in  this  respect  have  been  reversed,  and  60 
cycles  is  more  convenient,  giving  more  poles  at  the  same 
generator  speed,  and  so  less  power  per  pole. 

Sundry  odd  frequencies,  as  30  cycles,  33  cycles,  66  cycles, 
which  were  attempted  at  some  points,  especially  in  the  early 
days,  have  not  spread;  and  frequencies  below  25  cycles,  as  15 
cycles  and  8  cycles,  as  proposed  for  railroading,  have  not 
proved  of  sufficient  advantage — at  least  not  yet — so  that  in 
general,  in  the  design  of  an  electric  system,  only  the  two 
standard  frequencies,  25  and  60  cycles,  come  into  considera- 
tion. 

b.  Constant  current,  either  alternating  or  direct,  that  is, 
a  current  of  constant  amperage,  varying  in  voltage  with  the 
load,  is  mostly  used  for  street  lighting  by  arc  lamps;  for  all 
other  purposes,  constant  potential  is  employed. 


Pi 

1 2  GENERAL  LECTURES 

c.  For  long  distance  transmission,  the  highest  permis- 
sible voltage  is  used;  for  primary  distribution  by  alternating 
current,  2200  volts,  that  is,  voltages  between  2000  and  2600; 
for  alternating  current  secondary  distribution,  and  direct 
current  distribution,  220  to  260  volts,  and  for  direct  current 
railroading,  550  to  600  volts. 

i.    GENERAL  DISTRIBUTION  FOR  LIGHTING  AND  POWER. 

In  general  distribution  for  lighting  and  power,  direct 
current  and  60  cycles  alternating  current  are  available.  25 
cycles  alternating  current  is  not  well  suited,  since  it  does  not 
permit  arc  lighting,  and  for  incandescent  lighting  it  is  just  at 
the  limit ,  where  under  some  conditions  and  with  some  genera- 
tor waves,  flickering  shows,  while  with  others  it  does  not  show 
appreciably. 


Fi&.  1 

The  distribution  voltage  is  determined  by  the  limitation 
of  the  incandescent  lamp,  as  from  104  to  130  volts,  or  about 
no  volts,  no  volts  is  too  low  to  distribute  with  good  regu- 
lation, that  is,  with  negligible  voltage  drop,  any  appreciable 
amount  of  power,  and  so  practically  always  twice  that  voltage 
is  employed  in  the  distribution,  by  using  a  three-wire  system, 
with  no  volts  between  outside  and  neutral,  and  220  volts 
between  the  outside  conductors,  as  shown  diagrammatically  in 
Fig.  i.  By  approximately  balancing  the  load  between  the  two 
circuits,  the  current  in  the  neutral  conductor  is  very  small,  the 


GENERAL  REVIEW  13 

drop  of  voltage  so  negligible,  and  the  distribution,  regarding 
voltage  drop  and  copper  economy,  so  takes  place  at  220  volts, 
while  the  lamps  operate  at  no  volts.  Even  where  a  separate 
transformer  feeds  a  single  house,  usually  a  three-wire  distribu- 
tion is  preferable,  if  the  number  of  lamps  is  not  very  small. 

When  speaking  of  a  distribution  voltage  of  no,  some 
voltage  anywhere  in  the  range  from  104  to  130  volts  is 
employed.  Exactly  no  volts  is  rarely  used,  but  the  voltages 
of  distribution  systems  in  this  country  are  distributed  over 
the  whole  range,  so  as  to  secure  best  economy  of  the  incan- 
descent lamp. 

This  condition  was  brought  about  by  the  close  co-oper- 
ation, in  this  country,  between  the  illuminating  com- 
panies and  the  manufacturers  of  incandescent  lamps.  The 
constants  of  an  incandescent  lamp  are  the  candle  power — for 
instance  16;  the  economy — for  instance  3.1  watts  for  hori- 
zontal candle  power;  and  the  voltage — for  instance  no.  By 
careful  manufacture,  a  lamp  can  be  made  in  which  the  filament 
reaches  3.1  watts  per  candle  power  economy  at  16  c.  p.  within 
one-half  candle-power;  but  the  attempt  to  fulfill  at  the  same 
time  the  condition,  that  this  economy  and  candle  power  be 
reached  at  no  volts,  within  one-half  volt,  would  lead  to  a 
considerable  percentage  of  lamps  which  would  fall  outside  of 
the  narrow  range  permitted  in  the  deviation  from  the  three  con- 
stants; and  so,  if  the  same  distribution  voltage  were  used 
throughout  the  country,  either  a  much  larger  margin  of  varia- 
tion would  have  to  be  allowed  in  the  product,  that  is,  the 
lamps  would  be  far  less  uniform  in  quality — as  is  the  case 
abroad, — or  a  large  number  of  lamps  would  not  fulfill  the 
requirements,  could  not  be  used,  and  so  would  increase  the 
cost  of  the  rest. 


i4  GENERAL  LECTURES 

Therefore,  all  the  efforts  in  manufacture  are  con- 
centrated on  producing  the  specified  candle  power  at  the 
required  economy,  and  the  lamps  are  then  sorted  for  voltage. 
This  arrangement  scatters  the  lamps  over  a  considerable  voltage 
range,  and  different  voltages  are  then  adopted  by  different 
distribution  systems,  so  as  to  utilize  the  entire  product  of 
manufacture  at  its  maximum  economy.  The  result  of  this 
co-operation  between  lamp  manufacturers  and  users  is,  that 
the  incandescent  lamps  are  very  much  closer  to  requirements, 
and  more  uniform,  than  would  be  possible  otherwise.  The 
effect  however  is,  that  the  distribution  is  rarely  actually  no, 
and  in  alternating  current  systems,  the  primary  distribution 
voltage  not  2200,  but  some  voltage  in  the  range  between  2080 
and  2600,  as  in  step-down  transformers  a  constant  ratio  of 
transformation,  of  a  multiple  of  10  -f-  i,  is  always  used. 

In  the  following,  therefore,  when  speaking  of  no,  220 
or  2200  volts  in  distribution  systems,  always  one  of  the 
voltages  within  the  range  of  the  lamp  voltages  is  understood. 

In  (this  country,  no  volt  lamps  are  used  almost  exclu- 
sively, while  in  England,  for  instance,  the  220  volt  lamps  is 
generally  used,  in  a  three-wire  distribution  system  with  440 
volts  between  the  outside  conductors.  The  amount  of  copper 
required  in  the  distribution  system,  with  the  same  loss  of 
power  in  the  distributing  conductors,  is  inversely  proportional 
to  the  square  of  the  voltage.  That  is,  at  twice  the  voltage, 
twice  the  voltage  drop  can  be  allowed  for  the  same  distribution 
efficiency;  and  as  at  double  voltage  the  current  is  one-half,  for 
the  same  load  twice  the  voltage  drop  at  half  the  current  gives 
four  times  the  resistance,  that  is,  one-quarter  the  conductor 
material.  By  the  change  from  the  220  volt  distribution  with 
no  volt  lamps,  to  the  440  volt  distribution  with  220  volt 


GENERAL  REVIEW  15 

lamps,  the  amount  of  copper  in  the  distributing  conductor, 
and  thereby  the  cost  of  investment  can  be  greatly  reduced,  and 
current  supplied  over  greater  distances,  so  that  from  the  point 
of  view  of  the  economical  supply  of  current  at  the  customers' 
terminals,  the  higher  voltage  is  preferable.  However, 
in  the  usual  sizes,  from  50  to  60  watts  power  consump- 
tion and  so  16  candle  power  with  the  carbon  filament, 
and  correspondingly  higher  candle  power  with  the  more 
efficient  metallized  carbon  and  metal  filaments,  the  220  volt 
lamp  is  from  10  to  15%  less  efficient,  that  is,  requires  from 
10  to  15%  more  power  than  the  no  volt  lamp,  when  producing 
the  same  amount  of  light  at  the  same  useful  life.  This  differ- 
ence is  inherent  in  the  incandescent  lamp,  and  is  due  to  the  far 
greater  length  and  smaller  section  of  the  220  volt  filament, 
compared  with  the  no  volt  filament,  and  therefore  no  possibil- 
ity of  overcoming  it  exists ;  if  it  should  be  possible  to  build  a 
220  volt  1 6  candle  power  lamp  as  efficient — at  the  same  useful 
life  of  500  hours — as  the  present  no  volt  lamp,  this  would 
simply  mean,  that  by  the  same  improvement  the  efficiency  of 
the  110  volt  lamp  could  also  be  increased  from  10  to  15%,  and 
the  difference  would  remain.  For  smaller  units  than  16  candle 
power,  the  difference  in  efficiency  is  still  greater. 

This  loss  of  efficiency  of  10  to  15%,  resulting  from  the 
use  of  the  220  volt  lamp,  is  far  greater  than  the  saving  in 
power  and  in  cost  of  investment  in  the  supply  mains ;  and  the 
220  volt  system  with  no  volt  lamps  is  therefore  more  efficient, 
in  the  amount  of  light  produced  in  the  customer's  lamps,  than 
the  440  volt  system  with  220  volt  lamps.  In  this  country, 
since  the  early  days,  the  illuminating  companies  have  accepted 
the  responsibility  up  to  the  output  in  light  at  the  customer's 
lamps,  by  supplying  and  renewing  the  lamps  free  of  charge, 
and  the  system  using  no  volt  lamps  is  therefore  universally 


1 6  GENERAL  LECTURES 

employed  while  the  220  volt  lamp  has  no  right  to  existence; 
while  abroad,  where  the  supply  company  considers  its  responsi- 
bility ended  at  the  customer's  meter,  and  the  customer  is  left 
to  supply  his  own  lamps,  the  supply  company  saves  by  the  use 
of  440  volt  systems — at  the  expense  of  a  waste  of  power  in  the 
customer's  220  volt  lamps,  far  more  than  the  saving  effected 
by  the  supply  company. 

In  considering  distribution  systems,  it  therefore  is 
unnecessary  to  consider  any  other  lamp  voltage  than  no  volts 
(that  is,  the  range  of  voltage  represented  thereby). 

In  direct  current  distribution  systems,  as  used  in 
most  large  cities,  the  220  volt  network  is  fed  from  a  direct 
current  generating  station,  or — as  now  more  frequently  is 
the  case — from  a  converter  substation,  which  receives  ks  power 
as  three-phase  alternating,  usually  25  cycles,  from  the  main 
generating  station,  or  long  distance  transmission  line.  In 
alternating  current  distribution,  the  220  volt  distribution  cir- 
cuits are  fed  by  step-down  transformers  from  the  2200  volt 
primary  distribution  system.  In  the  latter  case,  where  con- 
siderable motor  load  has  to  be  considered,  some  arrangement 
of  polyphase  supply  is  desirable,  as  the  single-phase  motor  is 
inferior  to  the  polyphase  motor,  and  so  the  later  is  preferable 
for  large  and  moderate  sizes. 

COMPARISON  OF  ALTERNATING  CURRENT 
AND  DIRECT  CURRENT 

At  the  low  distribution  voltage  of  220,  current  can 
economically  be  supplfed  from  a  moderate  distance  only, 
rarely  exceeding  from  i  to  2  miles.  In  a  direct  current 
system,  the  current  must  be  supplied  from  a  generating  station 
or  a  converter  substation,  that  is,  a  station  containing  revolv- 
ing machinery.  As  such  a  station  requires  continuous  atten- 


GENERAL  REVIEW  17 

tion,  its  operation  would  hardly  be  economical  if  not  of  a 
capacity  of  at  least  some  hundred  kilowatts.  The  direct  cur- 
rent distribution  system  therefore  can  be  used  economically  only 
if  a  sufficient  demand  exists,  within  a  radius  of  i  to  2  miles,  to 
load  a  good  sized  generator  or  converter  substation.  The 
use  of  direct  current  is  therefore  restricted  to  those  places 
where  a  fairly  concentrated  load  exists,  as  in  large  cities; 
while  in  the  suburbs,  and  in  small  cities  and  villages,  where 
the  load  is  too  scattered  to  reach  from  one  low  tension 
supply  point,  sufficient  customers  to  load  a  substation,  the 
alternating  current  must  be  used,  as  it  requires  merely  a  step- 
down  transformer  which  needs  no  attention. 

In  the  interior  of  large  cities,  the  alternating  current 
system  is  at  a  disadvantage,  because  in  addition  to  the  voltage 
consumed  by  resistance,  an  additional  drop  of  vokage  occurs 
by  self-induction,  or  by  reactance;  and  with  the  large  conduc- 
tors required  for  the  distribution  of  a  large  low  (tension  current, 
the  drop  of  voltage  by  self-induction  is  far  greater  than  that  by 
resistance,  and  the  regulation  of  the  system  therefore  is  serious- 
ly impaired,  or  at  least  the  voltage  regulation  becomes  far  more 
difficult  than  with  direct  current.  A  second  disadvantage  of 
the  alternating  current  for  distribution  in  large  cities  is,  that 
a  considerable  part  of  the  motor  load  is  elevator  motors,  and 
the  alternating  current  elevator  motor  is  inferior  to  the  direct 
current  motor.  Elevator  service  essentially  consists  in  starting 
at  heavy  torque,  and  rapid  acceleration,  and  in  both  of  these 
features  the  direct  current  motor  with  compound  field  winding 
is  superior,  and  easier  to  control. 

Where  therefore  direct  current  can  be  used  in  low  tension 
distribution,  it  is  preferable  to  use  it,  and  to  relegate  alternat- 
ing current  low  tension  distribution  to  those  cases  where  direct 


1 8  GENERAL  LECTURES 

current  cannot  be  used,  that  is,  where  the  load  is  not  sufficiently 
concentrated  to  economically  operate  converter  substations. 

The  loss  of  power  in  the  low  tension  direct  current  system 
is  merely  the  i2r  loss  in  the  conductors,  which  is  zero  at  no 
load,  and  increases  with  the  load;  the  only  constant  loss  in 
a  direct  current  distribution  system  is  the  loss  of  power  in  the 
potential  coils  of  the  integrating  wattmeters  on  the  customer's 
premises.  In  the  direct  current  system  therefore,  the  efficiency 
of  distribution  is  highest  at  light  load,  and  decreases  with 
increasing  load. 

In  an  alternating  current  distribution  system,  with  a  2200 
volt  primary  distribution,  feeding  secondary  low  tension  cir- 
cuits by  step-down  transformers,  the  i*r  loss  in  the  conductors 
usually  is  far  smaller  than  in  the  direct  current  system,  but  a 
considerable  constant,  or  "no  load",  loss  exists;  the  core- 
loss  in  the  transformers,  and  the  efficiency  of  an  alternating 
current  distribution  is  usually  lowest  at  light  load,  but 
increases  with  increase  of  load,  since  with  increasing  load  the 
transformer  coreloss  becomes  a  lesser  and  lesser  percentage 
of  the  total  power.  The  i2r  loss  in  alternating  current  systems 
must  be  far  lower  than  in  direct  current  systems: 

1.  Because  it  is  not  the  only  loss,  and  the  existence  of 
the  "no  load"  or  transformer  coreloss  requires  to  reduce  the 
load  loss  or  i2r  loss,  if  an  equally  good  efficiency  is  desired. 
With  an  alternating  current  system,  each  low  tension  main 
requires  only  a  step-down  transformer,  which  needs  no  atten- 
tion ;  therefore  many  more  transformers  can  be  used  than  rotary 
converter  substations  in  a  direct  current  system,  and  the  i2r 
loss  is  then  reduced  by  the  greatly  reduced  distance  of  second- 
ary distribution. 

2.  In  the  alternating  current  system,  the  drop  of  voltage 
in  the  conductors  is  greater  by  the  self -inductive  drop  than  the 


GENERAL  REVIEW  19 

ir  drop ;  the  ir  drop  is  therefore  only  a  part  of  the  total  voltage 
drop ;  and  with  the  same  voltage  drop  and  therefore  the  same 
regulation  as  a  direct  current  system,  the  i2r  loss  in  the  alternat- 
ing current  system  would  be  smaller  -than  in  the  direct  current 
system. 

3.  Due  to  the  self-inductive  drop,  smaller  and  therefore 
more  numerous  low  tension  distribution  circuits  must  be  used 
with  alternating  current  than  with  direct  current,  and  a  separ- 
ate and  independent  voltage  regulation  of  each  low  tension  cir- 
cuit— that  is — each  transformer,  therefore  usually  becomes  im- 
practicable. This  means  -that  the  total  voltage  drop,  resistance 
and  inductance,  in  the  alternating  current  low  tension  distribu- 
tion circuits  must  be  kept  within  a  few  percent.,  that  is,  within 
the  range  permissible  by  the  incandescent  lamp.  As  a  result 
thereof,  the  voltage  regulation  of  an  alternating  current  low 
tension  distribution  is  usually  inferior  to  that  of  the  direct  cur- 
rent distribution — in  many  cases  to  such  an  extent  as  to  require 
the  use  of  incandescent  lamps  of  lower  efficiency.  While  there- 
fore in  direct  current  distribution  3.1  watt  lamps  are  always 
used,  in  many  alternating  current  systems  3.5  watt  lamps  have 
to  be  used,  as  the  voltage  regulation  is  not  suffiiently  good  to 
get  a  satisfactory  life  from  the  3.1  watt  lamps. 


SECOND  LECTURE 


GENERAL  DISTRIBUTION 

DIRECT  CURRENT  DISTRIBUTION 

HE  TYPICAL  direct  current  distribution  is  the  system 
of  feeders  and  mains,  as  devised  by  Edison,  and  since 
used  in  all  direct  current  distributions.     It  is  shown 
diagrammatically  in  Fig.  2.  The  conductors  are  usually  under- 

iii 


24  GENERAL  LECTURES 

ground,  as  direct  current  systems  are  used  only  in  large  cities. 
A  system  of  three-wire  conductors,  called  the  "mains"  is  laid 
down  in  the  streets  of  the  city,  shown  diagrammatically  by 
the  heavily  drawn  lines.  Commonly,  conductors  of  one  million 
circular  mill  section  (that  is,  a  copper  section  which  as  solid 
round  conductor  would  have  a  dia*neter  of  i")  are  used  for  the 
outside  conductors,  the  "positive"  and  the  "negative"  con- 
ductor; and  a  conductor  of  half  this  size  for  the  middle  or 
"neutral"  conductor.  The  latter  is  usually  grounded,  as  pro- 
tection against  fire  risk,  etc.  Conductors  of  more  than  one 
million  circular  mills  are  not  used,  but  when  the  load  exceeds 
the  capacity  of  such  conductors,  a  second  main  is  laid  down  in 
the  same  street.  A  number  of  feeders,  shown  by  dotted  lines 
in  Fig.  2,  radiate  from  the  generating  station  or  converter 
substations,  and  tap  into  the  mains  at  numerous  points ;  potential 
wires  run  back  from  the  mains  to  the  stations,  and  so  allow  of 
measuring,  in  the  station,  the  voltage  at  the  different  points  of 
the  distribution  system.  All  the  customers  are  connected  to  the 
mains,  but  none  to  the  feeders.  The  mains  and  feeders  are 
arranged  so  that  no  appreciable  voltage  drop  takes  place  in  the 
mains,  but  all  drop  of  voltage  occurs  in  the  feeders ;  and  as  no 
customers  connect  to  the  feeders,  the  only  limit  to  the  voltage 
drop  in  the  feeders  is  efficiency  of  distribution.  The  voltage  at 
the  feeding  points  into  the  mains  is  kept  constant  by  varying 
the  voltage  supply  to  the  feeders  with  the  changes  of  the  load  on 
the  mains.  This  is  done  by  having  a  number  of  outside 
bus  bars  in  the  station,  as  shown  diagrammaitically  in  Fig.  3, 
differing  from  each  other  in  voltage,  and  connecting  feeders 
over  from  bus  bar  to  bus  bar,  with  the  change  of  load. 

For  instance,  in  a  2  x  120  voltage  distribution,  the  station 
may  have,  in  addition  to  the  neutral  bus  bar  zero,  three  positive 


GENERAL  DISTRIBUTION 


bus  bars  i,  i',  i",  and  three  negative  bus  bars  2,  2',  2",  differing 
respectively  from  the  neutral  bus  by  120,  130  and  140  volts, 
as  shown  in  Fig.  3.  At  light  load,  when  the  drop  of  voltage 
in  the  feeders  is  negligible,  the  feeders  connect  to  the  busses 
i,  o,  2  of  1 20  volts.  When  .the  load  increases,  some  of  the 
feeders  are  shifted  over,  by  transfer  bus  bars,  to  the  130  volt 
busbars  i'  and  2';  with  still  further  increase  of  load,  more 
feeders  are  connected  over  to  130  volts;  then  some  feeders  are 
connected  to  the  140  volt  bus  bars,  i"  and  2",  and  so,  by  varying 


•z 
•# 

•2* 


t±d 


Fife.  3 

the  voltage  supply  to  the  feeders,  the  voltage  at  the  mains  can 
be  maintained  constant  with  an  accuracy  depending  on  the 
number  of  bus  bars.  It  is  obvious  that  a  shift  of  a  feeder  from 
one  voltage  to  another  does  not  mean  a  corresponding  voltage 
change  on  the  main  supplied  by  it,  but  rather  a  shift  of  load 
between  the  feeders,  and  so  a  readjustment  of  the  total  voltage 
in  the  territory  near  the  supply  point  of  the  feeder.  For 
instance,  if  by  the  potential  wires  a  drop  of  voltage  below  120 
volts  is  registered  in  the  main  at  the  connection  point  of  feeder 
A  in  Fig.  2,  and  this  feeder  then  shifted  from  the  supply 


26  GENERAL  LECTURES 

voltage  130  to  140,  the  current  in  the  main  near  A,  which 
before  flowed  towards  A  as  minimum  voltage  point,  reverses 
in  direction,  flows  away  from  A,  the  load  on  feeder  A  and  there- 
fore increases,  and  the  drop  of  voltage  in  A  increases,  while  the 
load  on  the  adjacent  feeders  decreases,  and  thereby  their  drop  of 
voltage  decreases,  with  the  result  of  bringing  up  the  voltage  in 
the  mains  at  the  feeder  A  and  all  adjacent  feeders.  This  inter- 
linkage  of  feeders  therefore  allows  a  regulation  of  voltage  in 
the  mains,  far  closer  than  the  number  of  voltages  available  in 
the  station. 

The  different  bus  bars  in  the  station  are  supplied  with  their 
voltage  by  having  different  generators  or  converters  in  the  sta- 
tion operate  at  different  voltages,  and  with  increasing  load  on 
the  station,  and  consequent  increasing  demand  of  higher  volt- 
age by  the  feeders ;  shift  machines  from  lower  to  higher  voltage 
bus  bars,  inversely  with  decreasing  load ;  or  the  different  bus 
bars  are  operated  through  boosters,  or  by  connection  with  the 
storage  battery  reserve,  etc. 

In  addition  to  feeders  and  mains,  tie  feeders  usually  con- 
nect the  generating  station  or  substation  with  adjacent  stations, 
so  that  during  periods  of  light  load,  or  in  case  of  breakdown, 
a  station  may  be  shut  down  altogether  and  supplied  from 
adjacent  stations  by  tie  feeders.  Such  tie  feeders  also  permit 
most  stations  to  operate  without  storage  battery  reserve,  that 
is,  to  concentrate  the  storage  batteries  in  a  few  stations,  from 
which  in  case  of  a  breakdown  of  the  system,  the  other  stations 
are  supplied  over  the  tie  feeders. 

ALTERNATING  CURRENT  DISTRIBUTION 

The  system  of  feeders  and  mains  allows  the  most  perfect 
voltage  regulation  in  the  distributing  mains.  It  is  however 
applicable  only  to  direct  current  distribution  in  a  territory  of 


GENERAL  DISTRIBUTION  27 

very  concentrated  load,  as  in  the  interior  of  a  large  city,  since 
the  independent  voltage  regulation  of  each  one  of  numerous 
feeders  is  economically  permissible  only  where  each  feeder 
represents  a  large  amount  of  power;  with  alternating  cur- 
rent systems,  the  inductive  drop  forbids  the  concentration  of 
such  large  currents  in  a  single  conductor.  That  is,  conductors 
of  one  million  circular  mills  cannot  be  used  economically  in 
an  alternating  current  system. 

The  resistance  of  a  conductor  is  inversely  proportional  to 
the  size  or  section  of  the  conductor,  hence  decreases  rapidly 
with  increasing  current:  a  conductor  of  one  million  circular 
mills  is  one-tenth  the  resistance  of  a  conductor  of  100,000 
circular  mills,  and  so  can  carry  ten  times  the  direct  current 
with  the  same  voltage  drop.  The  reactance  of  a  conductor, 
however,  and  so  the  voltage  consumed  by  self-induction,  de- 
creases only  very  little  with  the  increasing  size  of  a  conductor, 
as  seen  from  the  table  of  resistances  and  reactances  of 
conductors.  A  wire  No.  ooo  B  &  S  G  is  ten  times  the 
section  of  a  wire  No.  7,  and  therefore  one-tenth  the  resistance ; 
but  the  wire  No.  ooo  has  a  reactance  of  .109  ohms  per  1000 
feet,  the  wire  No.  7  has  a  reactance  of  .133  oms,  or  only  1.22 
times  as  large.  Hence,  while  in  the  wire  No.  7,  the  reactance, 
at  60  cycles,  is  only  .266  times  the  resistance  and  therefore  not 
of  serious  importance,  in  a  wire  No.  ooo  the  reactance  is  1.76 
times  the  resistance,  and  the  latter  conductor  is  likely  to  give 
a  voltage  drop  far  in  excess  of  the  ohmic  resistance  drop.  The 
ratio  of  reactance  and  resistance  therefore  rapidly  increases 
with  increasing  size  of  conductor,  and  for  alternating  currents, 
large  conductors  cannot  therefore  be  used  economically  where 
close  voltage  regulation  is  required. 

With  alternating  currents  it  therefore  is  preferable  to 
use  several  smaller  conductors  in  multiple :  two  conductors  of 


28  GENERAL  LECTURES 

No.  i  in  multiple  have  the  same  resistance  as  one  conductor 
of  No.  ooo;  but  the  reactance  of  one  conductor  No.  ooo  is  .109 
ohms,  and  so  1.88  times  as  great  as  the  reactance  of  two  con- 
ductors of  No.  i  in  multiple,  which  latter  is  half  that  of  one 
conductor  No.  i,  or  .058  ohms,  provided  that  the  two  con- 
ductors are  used  as  separate  circuits. 

In  alternating  current  low  tension  distribution,  the  size 
of  the  conductor  and  so  the  current  per  conductor,  is  limited 
by  the  self -inductive  drop,  and  alternating  current  low  tension 
networks  are  therefore  of  necessity  of  smaller  size  than  those 
of  direct  current  distribution. 

As  regards  economy  of  distribution,  this  is  not  a  serious 
objection,  as  the  alternating  current  transformer  and  primary 
distribution  permits  the  use  of  numerous  secondary  circuits. 

In  alternating  current  systems,  a  primary  distribution 
system  of  2200  volts  is  used,  feeding  step-down  transformers. 

The  different  arrangements  are — 

a.  A  separate  transformer  for  each  customer.  This  is 
necessary  in  those  cases  where  the  customers  are  so  far  apart 
from  each  other  that  they  cannot  be  reached  by  the  same  low 
tension  or  secondary  circuit ;  every  alternating  current  system 
therefore  has  at  least  a  number  of  instances  where  individual 
transformers  are  used. 

This  is  the  most  uneconomical  arrangement.  It  requires 
the  use  of  small  transformers,  which  are  necessarily  less 
efficient  and  more  expensive  per  kilowatt,  than  large  trans- 
formers. The  transformer  must  be  built  to  carry,  within  its 
overload  capacity,  all  the  lamps  installed  by  the  customer, 
since  all  the  lamps  may  be  used  occasionally.  Usually, 
however,  only  a  small  part  of  the  lamps  are  in  use,  and 
those  only  for  a  small  part  of  the  day ;  so  that  the  average 
load  on  the  transformer  is  a  very  small  part  of  its  capacity. 


GENERAL  DISTRIBUTION  29 

As  the  coreloss  in  the  transformer  continues  whether  the 
transformer  is  loaded  or  not,  but  is  not  paid  for  by  the  cus- 
tomer, the  economy  of  the  arrangement  is  very  low ;  and  so  it 
can  be  understood  that  in  the  early  days,  where  this  arrange- 
ment was  generally  used,  the  financial  results  of  most  alternat- 
ing current  distributions  were  very  discouraging. 

Assuming  as  an  instance  a  connected  load  of  20  16  candle 
power  lamps — low  efficiency  lamps,  of  60  watts  per  lamp,  since 
the  voltage  regulation  cannot  be  very  perfect — allowing  then 
in  cases  of  all  lamps  being  used,  an  overload  of  100%,  which 
is  rather  beyond  safe  limits,  and  permissible  only  on  the 
assumption  that  this  load  will  occur  very  rarely,  and  for  a 
short  time — the  transformer  would  have  600  watt  rating. 
Assuming  a  coreloss  of  4%,  this  gives  a  continuous  power 
consumption  of  24  watts.  Usually  probably  only  one  or  two 
lamps  will  be  burning,  and  these  only  a  few  hours  per  day, 
so  that  the  use  of  two  lamps,  at  an  average — summer  and 
winter — of  three  hours  per  day,  would  probably  be  a  fair 
example  of  many  such  cases.  Two  lamps  or  120  watts,  for 
three  hours  per  day,  give  an  average  power  of  15  watts, 
which  is  paid  for  by  the  customer,  while  the  continuous  loss  in 
the  transformer  is  24  watts ;  so  that  the  all  year  efficiency,  or  the 
ratio  of  the  power  paid  for  by  the  customer,  to  the  power  con- 

15 

sumed  by  the  transformer,  is  only or  38%. 

15  +  24 

By  connecting  several  adjacent  customers  to  the  same 
transformer,  the  conditions  immediately  become  far  more 
favorable.  It  is  extremely  improbable  that  all  the  customers 
will  burn  all  their  lamps  at  the  same  time,  the  more  so,  the 
greater  the  number  of  customers  is,  which  are  supplied  from 
the  same  transformer.  It  therefore  becomes  unnecessary  to 


30  GENERAL  LECTURES 

allow  a  transformer  capacity  capable  of  operating  all  the  con- 
nected load.  The  larger  transformer  also  has  a  higher 
effiicency.  Assuming  therefore  as  an  instance,  four  customers 
of  20  lamps  connected  load  each.  The  average  load  would  be 
about  8  lamps.  Assuming  even  one  customer  burning  all  20 
lamps,  it  is  not  probable  that  the  other  customers  together 
would  at  this  time  burn  more  than  10  to  15  lamps,  and  a  trans- 
former carrying  30  to  35  lamps  at  overload  would  probably 
be  sufficient.  A  1500  watt  transformer  would  therefore  be 
larger  than  necessary.  At  3%  coreloss,  this  gives  a  constant 
loss  of  45  watts,  while  an  average  load  of  8  lamps  for  3  hours 
per  day  gives  a  useful  output  of  60  watts,  or  an  all  year 
efficiency  of  nearly  60%,  while  a  1000  watt  transformer  would 
give  an  all  year  efficiency  of  67%. 

This  also  illustrates  that  in  smaller  transformers  a  low 
coreloss  is  of  utmost  importance,  while  the  i2r  loss  is  of  very 
secondary  importance,  since  it  is  appreciable  only  at  heavy  load, 
and  therefore  affects  the  all  year  efficiency  very  little. 

When  it  becomes  possible  to  connect  a  large  number  of 
customers  to  a  secondary  main  fed  from  one  large  trans- 
former the  connected  load  ceases  to  be  of  moment  in  the  trans- 
former capacity ;  the  transformer  capacity  is  determined  by  the 
average  load,  with  a  safe  margin  for  overloads;  in  this  case, 
good  all  year  efficiencies  can  be  reached. 

Economical  alternating  current  distribution  therefore  re- 
quires the  use  of  secondary  distribution  mains  of  as  large  an 
extent  as  possible,  fed  by  large  transformers.  The  distance, 
however,  to  which  a  transformer  can  supply  secondary  current, 
is  rather  limited  by  the  inductive  drop  of  voltage ;  therefore,  for 
supplying  secondary  mains,  transformers  of  larger  size  than  30 
kw.  are  rarely  used,  but  rather  several  transformers  are  em- 
ployed, to  feed  in  the  same  main  at  different  points. 


GENERAL  DISTRIBUTION  31 

Extending  the  secondary  mains  still  further  by  the  use 
of  several  transformers  feeding  into  the  same  mains,  or,  as  it 
may  be  considered,  inter-connecting  the  secondary  mains  of  the 
different  transformers,  we  arrive  at  a  system  somewhat  similar 
to  the  direct  current  system :  a  low  tension  distribution  system 
of  220  volts  three-wire  mains,  with  a  system  of  feeders  tapping 
into  it  at  a  number  of  points,  as  shown  in  Fig.  4.  These  feeders 


Fl&.  4.    Alternating  Current  Distribution  with  Secondary 
Mains  and  Primary  Feeders. 

are  primary  feeders  of  2200  volts,  connecting  to  the  mains 
through  step-down  transformers.  In  such  a  system,  by  vary- 
ing the  voltage  impressed  upon  the  primary  feeders,  a  voltage 
regulation  of  the  system  similar  to  that  of  direct  current  dis- 
tribution becomes  feasible.  Such  an  arrangement  has  these 
advantages  over  the  direct  current  system:  the  drop  in  the 
feeders  is  very  much  lower,  due  to  their  higher  voltage;  and 


32  GENERAL  LECTURES 

that  the  feeder  voltage  can  be  regulated  by  alternating  current 
feeder  regulators  or  compensators,  that  is,  stationary  structures 
similar  to  the  transformer.  It  has,  however,  the  disadvantage 
that,  due  to  the  self-induction  of  the  mains,  each  feeding  point 
can  supply  current  over  a  far  shorter  distance  than  with 
direct  current,  and  the  interchange  of  current  between 
feeders,  by  which  the  load  can  be  shifted  and  apportioned 
between  the  feeders,  is  far  less. 

As  a  result,  it  is  difficult  to  reach  as  good  voltage  regu- 
lation with  the  same  attention  to  the  system;  and  since 
this  arrangement  has  the  disadvantage  -that  any  break- 
down in  the  secondary  system  or  in  a  transformer  may 
involve  the  entire  system,  this  system  of  inter-connected 
secondary  mains  is  rarely  used  for  alternating  current 
distribution,  but  the  secondary  mains  are  usually  kept 
separate.  That  is,  as  shown  diagrammadcally  in  Fig.  5,  a 
number  of  separate  secondary  mains  are  fed  by  large  trans- 
formers from  primary  feeders,  and  usually  each  primary 
feeder  connects  to  a  number  of  transformers.  Where  the 
distances  are  considerable,  and  the  voltage  drop  in  the  primary 
feeders  appreciable,  voltage  regulation  of  the  feeders  becomes 
necessary ;  and  in  this  case,  to  get  good  voltage  regulation  in  the 
system,  attention  must  be  given  to  the  arrangements  of  the 
feeders  and  mains.  That  is,  all  the  transformers  on  the  same 
feeder  should1  be  at  about  the  same  distance  from  the  station, 
so  that  the  voltage  drop  between  the  transformers  on  the  same 
feeder  is  negligible;  and  the  nature  of  the  load  on  the  secondary 
mains  fed  by  the  same  feeder  should  be  about  as  nearly  the 
same  as  feasible,  so  that  all  the  mains  on  the  same  feeder  are 
about  equally  loaded.  It  would  therefore  be  undesirable  for 
voltage  regulation,  to  connect,  for  instance,  a  main  feeding  a 


GENERAL  DISTRIBUTION 


33 


residential  section  to  the  same  feeder  as  a  main  feeding  a 
business  district  or  an  office  building. 


~i 


i_ 


Fi£.  5.    Typical  Alternating  Current  Distribution. 

In  a  well  designed  alternating  current  distribution  system, 
that  is,  a  system  using  secondary  distribution  mains  as  far  as 
feasible,  the  all  year  efficiency  is  about  the  same  as  with  the 
direct  current  system.  In  such  an  alternating  current  system, 


34  GENERAL  LECTURES 

the  efficiency  at  heavy  load  is  higher,  and  at  light  load  lower, 
than  in  the  direct  current  system ;  in  this  respect  the  alternating 
current  system  has  the  advantage  over  the  direct  current 
system,  since  at  the  time  of  heavy  load  the  power  is  more 
valuable  than  at  light  load. 


THIRD  LECTURE 


LIGHT  AND  POWER  DISTRIBUTION 

N  A  DIRECT  current  distribution  system,  the  motor 
load  is  connected  to  the  outside  mains  at  220  volts, 
and  only  very  small  motors,  as  fan  motors,  between 
outside  mains  and  neutral ;  since  the  latter  connection,  with  a 
large  motor,  would  locally  unbalance  a  system.  The  effect  of 
a  motor  on  the  system  depends  upon  its  size  and  starting 
current,  and  with  the  large  mains  and  feeders,  which  are  gener- 
ally used,  even  the  starting  of  large  elevator  motors  has  no 
appreciable  effect,  and  the  supply  of  power  to  electric  elevators 
represents  a  very  important  use  of  direct  current  distribution. 

In  alternating  current  distribution  systems,  the  effect  on 
the  voltage  regulation,  when  starting  a  motor,  is  far  more 
severe;  since  alternating  current  motors  in  starting  usually 
take  a  larger  current  than  direct  current  motors  starting  with 
the  same  torque  on  the  same  voltage;  and  the  current  of  the 
alternating  current  motor  is  lagging,  the  voltage  drop  caused 
by  it  in  the  reactance  is  therefore  far  greater  than  would  be 
caused  by  the  same  current  taken  by  a  non-inductive  load,  as 
lamps.  Furthermore,  alternating  current  supply  mains  usually 
are  of  far  smaller  capacity,  and  therefore  more  affected  in 
voltage.  Large  motors  are  therefore  rarely  connected  to  the 
lighting  mains  of  an  alternating  current  system,  but  separate 
transformers  and  frequently  separate  feeders  are  used  for  the 
motors,  and  very  large  motors  commonly  built  for  the  primary 
distribution  voltage  of  2200,  are  connected  to  these  mains. 

For  use  in  an  alternating  current  distribution  system,  the 
synchronous  motor  hardly  comes  into  consideration,  since  the 
synchronous  type  is  suitable  mainly  for  large  powers,  where 
it  is  operated  on  a  separate  circuit. 


38  GENERAL  LECTURES 

The  alternating  current  motor  mostly  used  in  small  and 
moderate  sizes — such  as  come  into  consideration  for  power 
distribution  from  a  general  supply  system — is  the  induction 
motor.  The  single-phase  induction  motor,  however,  is  so 
inferior  to  the  polyphase  induction  motor,  -that  single-phase 
motors  are  used  only  in  small  sizes;  for  medium  and  larger 
sizes  the  three-phase  or  two-phase  motor  is  preferred.  This 
however,  introduces  a  complication  in  the  distribution  system, 
and  the  three-wire  single-phase  system  therefore  is  less  suited 
for  motor  supply,  but  additional  conductors  have  to  be  added 
to  give  a  polyphase  power  supply  to  the  motor.  As  the  result 
thereof,  motors  are  not  used  in  alternating  current  systems  to 
the  same  extent  as  in  direct  current  systems.  In  the  alternat- 
ing current  system,  however,  the  motor  load  is,  if  anything, 
more  important  than  in  the  direct  current  system,  to  increase 
the  load  factor  of  the  system ;  since  the  efficiency  of  the  alter- 
nating current  system  decreases  with  decrease  of  load,  while 
that  of  a  direct  current  system  increases. 

Compared  with  the  direct  current  motor,  the  polyphase 
induction  motor  has  the  disadvantage  of  being  less  flexible: 
its  speed  cannot  be  varied  economically,  as  that  of  a  direct 
current  motor  by  varying  the  field  excitation.  Speed  variation 
of  the  induction  motor  produced  by  a  rheostat  in  the  armature 
or  secondary  circuit,  in  the  so-called  form  "M"  motor  is 
accomplished  by  wasting  power :  the  power  input  of  an  induc- 
tion motor  always  corresponds  to  full  speed;  if  the  speed 
is  reduced  by  running  on  the  rheostat,  the  difference  in 
power  between  that  which  the  motor  actually  gives,  and  that 
which  it  would  give,  with  the  same  torque,  at  full  speed,  is 
consumed  in  the  rheostat. 

Where  therefore  different  motor  speeds  are  required,  pro- 
visions are  made  in  the  induction  motor  to  change  the  number 


LIGHT  AND  POWER  DISTRIBUTION          39 

of  poles;  thereby  a  number  of  different  definite  speeds  are 
available,  at  which  the  motor  operates  economically  as  "multi- 
speed"  motor. 

The  starting  torque  of  the  polyphase  induction  motor 
with  starting  rheostat  in  the  armature  (Form  L,  motor)  is 
the  same  as  the  running  torque  at  the  same  current  input,  just 
as  in  the  case  of  the  direct  current  shunt  motor  with  constant 
field  excitation.  In  the  squirrel  cage  induction  motor,  how- 
ever, (form  K  motor)  the  starting  torque  is  far  less  than  the 
running  torque  at  the  same  current  input;  or  inversely,  to 
produce  the  same  starting  torque,  a  greater  starting  current  is 
required.  In  starting  torque  or  current,  the  squirrel  cage 
induction  motor  has  the  disadvantage  against  the  direct 
current  motor.  It  has,  however,  an  enormous  advantage  over 
it  in  its  greater  simplicity  and  reliability,  due  to  the  absence 
of  commutator  and  brushes,  and  the  use  of  a  squirrel  cage 
armature. 

The  advantage  of  simplicity  and  reliability  of  the  squir- 
rel cage  induction  motor  sufficiently  compensates  for  the 
disadvantage  of  the  large  starting  current,  to  make  the  motor 
most  commonly  used.  In  an  alternating  current  distribution 
system,  however,  great  care  has  to  be  taken  to  avoid  the  use 
of  such  larger  motors  at  places  where  their  heavy  lagging 
starting  currents  may  affect  the  voltage  regulation;  in  such 
places,  separate  transformers  and  even  separate  primary 
feeders  are  desirable. 

The  single-phase  induction  motor  is  not  desirable  in  larger 
sizes  in  a  distribution  system,  since  its  starting  current  is  still 
larger;  in  small  sizes,  however,  it  is  extensively  used,  since 
it  requires  no  special  conductors,  but  can  be  operated  from  a 
single-phase  lighting  main. 


40  GENERAL  LECTURES 

The  alternating  current  commutator  motor  is  a  single- 
phase  motor  which  has  all  the  advantages  of  the  different 
types  of  direct  current  motors;  it  can  be  built  as  constant 
speed  motor  of  the  shunt  type,  or  as  motor  with  the  charac- 
teristics of  the  direct  current  series  motor :  very  high  starting 
torque  with  moderate  starting  current.  It  has,  however,  also 
the  disadvantages  of  the  direct  current  motor:  commutator 
and  brushes;  and  so  requires  more  attention  than  the  squirrel 
cage  induction  motor. 

Alternating  current  generators  now  are  almost  always 
used  as  polyphase  machines,  three-phase  or  two-phase,  and 
transmission  lines  are  always  three-phase,  though  in  transform- 
ing down,  the  system  can  be  changed  to  two-phase.  The  power 
supply  in  an  alternating  current  system  therefore  is  practically 
always  polyphase;  and  since  a  motor  load,  which  is  very  desir- 
able for  economical  operation,  also  requires  polyphase  currents, 
alternating  current  distribution  systems  always  start  from  poly- 
phase power. 

The  problem  of  alternating  current  distribution  therefore 
is  to  supply,  from  a  polyphase  generating  system,  single-phase 
current  to  the  incandescent  lamps,  and  polyphase  current  to  the 
induction  motors. 

PRIMARY  DISTRIBUTION  SYSTEMS 

i.  Two  conductors  of  the  three-phase  generating  or 
transmission  system  are  used  to  supply  a  2200  single-phase 
system  for  lighting  by  step-down  transformers  and  three-wire 
secondary  mains ;  the  third  conductor  is  carried  to  those  places 
where  motors  are  used  and  three-phase  motors  are  operated 
by  separate  step-down  transformers.  In  the  lighting  feeders, 
the  voltage  is  then  controlled  by  feeder  regulators,  or,  in  a 
smaller  system,  the  generator  excitation  is  varied  so  as  to  main- 


LIGHT  AND  POWER  DISTRIBUTION          41 

tain  the  proper  voltage  on  the  lighting  phase.  At  load,  the 
three-phase  triangle  then  more  or  less  unbalances,  but  induction 
motors  are  very  little  sensitive  to  unbalancing  of  the  voltage, 
and  by  their  regulation — by  taking  more  current  from  the 
phase  of  higher,  less  from  the  phase  of  lower  voltage — tend 
to  restore  the  balance.  For  smaller  motors,  frequently  two 
transformers  are  used,  arranged  in  "open  delta"  connection. 

2.  Two-phase  generators  are  used,  or  in  the  step-down 
transformers  of  a  three-phase  transmission  line,  the  voltage  is 
changed  from  three-phase  to  two-phase;  the  lighting  feeders 
are  distributed  between  the  two  phases  and  controlled  by  poten- 
tial regulators  so  that  the  distribution  for  lighting  is  single- 
phase,  by  three-wire  secondary  mains.   For  motors,  both  phases 
are  brought  together,  and  the  voltage  stepped  down  for  use  on 
two-phase  motors.    This  requires  four,  or  at  least  three,  prim- 
ary wires  to  motor  loads. 

3.  From  three-phase   generators   or   transmission   lines, 
three  separate  single-phase  systems  are  operated  for  lighting; 
that  is  the  lighting  feeders  are  distributed  between  the  three 
hases,  and  all  three  primary  wires  are  brought  to  the  step-down 
transformers  for  motors.  This  arrangement,  by  distributing  the 
lighting  feeders  between  the  three  phases,  would  require  more 
care  in  exactly  balancing  the  load  between   all    three   phases 
than  two,  but  a  much   greater  unbalancing  can   be  allowed 
without  affecting  the  voltage. 

4.  Four-wire    three-phase    primary    distribution    with 
grounded  neutral,  and  2200  volts  between  outside  conductors 
and  neutral.     The  lighting  feeders  are  distributed  between 
<the  three  circuits  between  outside  conductors  and  neutral,  and 
motors  supplied  by  three  of  such  transformers.    This  system  is 
becoming  of  increasing  importance,  since  it  allows  economical 
distribution  to  distances  beyond  those  which  can  be  reached 


42  GENERAL  LECTURES 

with  2200  volts :  with  2600  volts  on  the  transformers — as  the 
upper  limit  of  primary  distribution  voltage — the  voltage  be- 
tween outside  conductors  is  4500,  and  the  copper  economy  of 
the  system  therefore  is  that  of  a  4500  volt  three-phase  system. 
5.  Polyphase  primary  and  polyphase  secondary  distri- 
bution, with  the  motor  connected  to  (the  same  secondary  mains 
as  the  lights. 

SYSTEMS  OF  LOW  TENSION  DISTRIBUTION  FOR 
LIGHTING  AND  POWER. 

i.     Two- WIRE   DIRECT    CURRENT   OR    SINGLE-PHASE    no 

VOLTS.    Fig.  6,  Cu.  i. 

This  can  can  be  used  only  for  very  short  distances,  since 
its  copper  economy  is  very  low,  that  is,  the  amount  of  conduc- 
tor material  is  very  high  for  a  given  power. 


O 


s> 
* 

O 


LIGHT  AND  POWER  DISTRIBUTION 


43 


2.     THREE- WIRE  DIRECT  CURRENT  OR  SINGLE-PHASE  no- 
220  VOLTS.    Fig.  7. 

Neutral  one-half  size  of  the  -two  outside  conductors.  The 
two  outside  conductors  require  one-quarter  the  copper  of  the 
two  wires  of  a  no  volt  system;  since  at  twice  the  voltage  and 
one-half  the  current,  four  times  the  resistance  or  one-quarter 


//tf/K 


/TO* 


JUO* 


Jo* 


the  copper  is  sufficient  for  the  same  loss  (the  amount  of  con- 
ductor material  varying  with  the  square  of  the  voltage). 

Adding  then  one-quarter  for  the  neutral  of  half-size, 
gives  V4  x  V4  =  V16  or  altogether  V*  +  V16  =  Vi«  of  the 
conductor  material  required  by  the  two- wire  no  volt 
system.  That  is,  the  copper  economy  is  5/16.  This  is  the 
most  commonly  used  system,  since  it  is  very  economical,  and 
requires  only  three  conductors.  It  is,  however,  a  single-phase 


44 


GENERAL  LECTURES 


system,  and  therefore  not  suitable  for  operating  polyphase  in- 
duction motors. 

Cu.  V« 

3.     FOUR- WIRE:  QUARTER-PHASE:  (TWO-PHASE).    Fig.  8. 

Two  separate  two-wire  single-phase  circuits,  therefore 
no  saving  in  copper  over  two-wire  systems.  That  is,  the  cop- 
per economy  is :  Cu.  i. 


o 

u      t 

o 

0 

r       i 

4.     THREE- WIRE  QUARTER-PHASE.    Fig.  9. 

Common  return  of  both  phases,  therefore  saves  one  wire 
or  one-quarter  of  the  copper;  hence  has  the  copper  economy: 
Cu.  8A. 


/</<?. 


$.     THREE- WIRE  THREE-PHASE.    Fig.  10. 

A  three-phase  system  is  best  considered  as  a  combination  of 
three  single-phase  systems,  of  the  voltage  from  line  to  neutral, 


LIGHT  AND  POWER  DISTRIBUTION 


45 


and  with  zero  return  (because  the  three  currents  neutralize 
each  other  in  the  neutral). 

Compared  thereto  the  two-wire  single-phase  system  can 
be  considered  as  a  combination  of  two  single-phase  circuits 
from  wire  to  neutral  with  zero  return. 


/=*/<?./*? 


In  a  no  volt  single-phase  system  the  voltage  from  line  to 
neutral  equals  "%,  in  a  three-phase  system  equals 


110 

1 ' 


The  ratio  of  voltages  is  110/2  H-110/-/*  ,  or  - 

3  X  110 

and  the  square  of  the  ratio  of  voltages  equals  8A ;  and  as  the 
copper  economy  varies  with  the  square  of  the  voltage,  the 
copper  economy  for  the  three-wire  three-phase  system  is : 

Cu.  8A. 

6.     FIVE- WIRE  QUARTER-PHASE.     Fig.  u. 

Neglecting  the  neutral  conductor,  the  five-wire  quarter- 
phase  system  can  be  considered  as  four  single-phase  circuits 
without  return,  from  line  to  neutral,  of  voltage  no.  Com- 
pared with  the  two-wire  circuit,  which  consists  of  two  single- 
phase  circuits  without  return,  of  110A  volts,  No.  6  therefore 
has  twice  the  voltage  of  No.  i ;  therefore  one-quarter  the 
copper. 


4*  GENERAL  LECTURES 

Making  the  neutral  half  the  size  of  the  main  conductor 
adds  one-half  of  the  copper  of  one  conductor,  or  1/8  of  Y* 


/ 


/I 


\ 


//o 


s/o 


\1  / 


=  Vsz,  so  giving  a  total  of  Y*  +  V32,  that  is,  a  copper  economy 
ofrCu.  =  •/«. 

7.     FOUR- WIRE  THREE-PHASE:.    Fig.  12. 

Lamps  connected  between  line  and  neutral. 
Neglecting  the  neutral,  the  system  consists  of  three  single- 
phase  circuits  without  return,  of  no  volts,  and  compared  with 


T 


I          jr         //fcfcy 

T 


?.  12 


the  two-wire  circuit  of  110/2  between  wire  and  neutral  without 
return,  it  therefore  requires  one-quarter  the  copper. 

Making  the  neutral  one-half  size  adds  Ve  of  the  copper, 
or  ye  of  Y*  —  Y24,  and  so  gives  a  total  copper  economy  of 
Vu  +  V*  =  VM.  Cu.  =  Y2*. 


LIGHT  AND  POWER  DISTRIBUTION 


47 


8.     THREE-WIRE  SINGLE-PHASE  LIGHTING  WITH   THREE- 
PHASE  POWER.    Fig.  13. 
Lighting:    Half  size  neutral,  same  as  No.  2,  therefore 

copper  economy :  Cu.  =  5/i«. 

Power:    Three-wire  three-phase  220  volts;  that  is,  the 

same  as  No.  5,  but  twice  the  voltage,  thus  one-quarter  the 

copper  of  No.  5,  or  V4  of  SA  =  Vie :     Cu.  =  •/«- 


o 
o 
a 

0 

a 

! 

0 

*            J 

f 

£20 

A 

o     . 

^ 

T 

AUM 

! 

o 
o 

o 
o 

0 

«=> 

/*<?. 


The  systems  mostly  used  are  : 

No.  2.  Three-wire  direct  current  or  alternating  current 
single-phase. 

No.  8.  Three-wire  lighting,  three-phase  power.  Less 
frequent. 

No.  6.     Fire-wire  quarter-phase. 

No.  7.     Four-wire  three-phase. 

As  we  have  seen,  the  two-wire  system  is  rather  inefficient 
in  copper.  High  efficiency  requires  the  use  of  a  third  conduc- 
tor, that  is,  the  three-wire  system,  for  direct  current  or  single- 
phase  alternating  current. 

Three-wire  polyphase  systems,  however,  are  inefficient  in 
copper,  as  No.  4  and  No.  5;  and  to  reach  approximately  the 
same  copper  economy,  as  is  reached  by  a  three-wire  system 
with  direct  current  and  single-phase  alternating  current,  re- 
quires at  least  four  wires  with  a  polyphase  system. 


48  GENERAL  LECTURES 

That  is,  for  equal  economy  in  conductor  material,  the 
polyphase  system  requires  at  least  one  more  conductor  than 
the  single-phase  or  the  direct  current  distribution  system. 

While  the  field  of  direct  current  distribution  is  found 
in  the  interior  of  large  cities,  alternating  current  is  used  in 
smaller  towns  and  villages  and  in  the  suburbs  of  large  cities. 
In  the  latter,  therefore,  alternating  current  does  the  pioneer 
work.  That  is,  the  district  is  developed  by  alternating  current, 
usually  with  overhead  conductors,  and  when  the  load  has  be- 
come sufficiently  large  to  warrant  the  establishment  of  con- 
verter substations,  direct  current  underground  mains  and  feed- 
ers are  laid  down  under  ground,  the  alternating  current  distri- 
bution is  abandoned,  and  the  few  alternating  current  motors 
are  replaced  by  direct  current  motors.    In  the  last  years,  how- 
ever, considerable  motor  load  has  been  developed  in  the  alter- 
nating current  suburban  distribution  systems,  fairly  satisfactor- 
ily alternating  current  elevator  motors  have  been  developed  and 
introduced  and  the  motor  load  has  become  so  large  as  to  make 
it  economically  difficult  to   replace  the   alternating  current 
motors  by  direct  current  motors  in  changing  the  system  to 
direct  current;  and  it  therefore  appears  that  the  distribution 
systems  of  large  cities  will  be  forced  to  maintain  alternating 
Current  distribution  even  in  districts  of  such  character  as  would 
make  direct  current  preferable. 


FOURTH  LECTURE 


LOAD  FACTOR  AND  COST  OF  POWER 

The  cost  of  the  power  supplied  at  the  customer's  meter 
consists  of  three  parts. 

A.  A  fixed  cost,  that  is,  cost  which  is  independent  of  the 
amount  of  power  used,  or  the  same  whether  the  system  is  fully 
loaded  or  carries  practically  no  load.     Of  this  character,  for 
instance,  is  the  interest  on  the  investment  in  the  plant,  the 
salaries  of  its  officers,  etc. 

B.  A  cost  which  is  proportional  to  the  amount  of  power 
used.  Such  a  proportional  cost,  for  instance,  is  that  of  fuel  in  a 
steam  plant. 

C.  A  cost  depending  on  the  reliability  of  service  required, 
as  the  cost  of  keeping  a  steam  reserve  in  a  water  power  trans- 
mission, or  a  storage  battery  reserve  in  a  direct  current  dis- 
tribution. 

Since  of  the  three  parts  of  the  cost,  only  one,  B,  is  propor- 
tional to  the  power  used,  hence  constant  per  kilowatt  output, — 
the  other  two  parts  being  independent  of  the  output, — hence 
the  higher  per  kilowatt,  the  smaller  a  part  of  the  capacity  of  the 
plant  the  output  is ;  it  follows  that  the  cost  of  power  delivered 
is  a  function  of  the  ratio  of  the  actual  output  of  the  plant,  to 
the  available  capacity. 

Interest  on  the  investment  of  developing  the  water  power 
or  building  the  steam  plant,  the  transmission  lines,  cables  and 
distribution  circuits,  and  depreciation  are  items  of  the  character 
A,  or  fixed  cost,  since  they  are  practically  independent  of  the 
power  which  is  produced  and  utilized. 

Fuel  in  a  steam  plant,  oil,  etc.,  are  proportional  costs,  that 
is,  essentially  depending  on  the  amount  of  power  produced. 


52  GENERAL  LECTURES 

Salaries  are  fixed  cost,  A ;  labor,  attendance  and  inspection 
are  partly  fixed  cost  A,  partly  proportional  cost  B, — economy 
of  operation  requires  therefore  a  shifting  of  as  large  a  part 
thereof  over  into  class  B,  by  shutting  down  smaller  substations 
during  periods  of  light  load,  etc. 

Incandescent  lamp  renewals,  arc  lamp  trimming,  etc.,  are 
essentially  proportional  costs,  B. 

The  reserve  capacity  of  a  plant,  the  steam  reserve  of  the 
transmission  line,  the  difference  in  cost  between  a  duplicate 
pole  line  and  a  single  pole  line  with  two  circuits,  the  storage 
battery  reserve  of  the  distribution  system,  the  tie  feeders 
between  stations,  etc.,  are  items  of  the  character  C ;  that  is,  part 
of  the  cost  insuring  the  reliability  and  continuity  of  power 
supply. 

The  greater  the  fixed  cost  A  is,  compared  with  the  propor- 
tional cost  B,  the  more  rapidly  the  cost  of  power  per  kilowatt 
output  increases  with  decreasing  load.  In  steam  plants  very 
frequently  A  is  larger  than  B,  that  is,  fuel,  etc.  not  being  the 
largest  items  of  cost;  in  water  power  plants  A  practically  al- 
ways is  far  larger  than  B.  As  result  thereof,  while  water  power 
may  appear  very  cheap  when  considering  only  the  proportional 
cost  B — which  is  very  low  in  most  water  powers — the  fixed 
cost  A  usually  is  very  high,  due  to  the  hydraulic  development 
required.  The  difference  in  the  cost  of  water  power  from  that 
of  steam  power  therefore  is  far  less  than  appears  at  first.  As 
water  power  is  usually  transmitted  over  a  long  distance  line, 
while  steam  power  is  generated  near  the  place  of  consumption, 
water  power  usually  is  far  less  reliable  than  steam  power.  To 
insure  equal  reliability,  a  water  power  plant  brings  the  item  C, 
the  reliability  cost,  very  high  in  comparison  with  the  reliability 
cost  of  a  steam  power  plant,  since  the  possibility  of  a  break- 
down of  a  transmission  line  requires  a  steam  reserve,  and 


LOAD  FACTOR  AND  COST  OF  POWER        53 

where  absolute  continuity  of  service  is  required,  it  requires  also 
a  storage  battery,  etc. :  so  that  on  the  basis  of  equal  reliability  of 
service,  sometimes  very  little  difference  in  cost  exists  between 
steam  power  and  water  power,  unless  the  hydraulic  develop- 
ment of  the  latter  was  very  simple. 

The  cost  of  electric  power  of  different  systems  therefore 
is  not  directly  comparable  without  taking  into  consideration 
the  reliability  of  service  and  the  character  of  the  load. 

As  a  very  large,  and  frequently  even  the  largest  part  of 
the  cost  of  power,  is  independent  of  the  power  utilized,  and 
therefore  rapidly  increases  with  decreasing  load  on  the  system, 
the  ratio  of  average  power  output  to  the  available  power  capac- 
ity of  the  plant  is  of  fundamental  importance  in  the  cost  of 
power  per  kilowatt  delivered.  This  ratio,  of  the  average 
power  consumption  to  the  available  power,  or  station  capacity, 
has  occasionally  been  called  "load  factor."  This  definition  of 
the  term  "load  factor"  is,  however,  undesirable,  since  it  does 
not  take  into  consideration  the  surplus  capacity  of  the  station, 
which  may  have  been  provided  for  future  extension;  the 
reserve  for  insuring  reliability  C,  etc. ;  and  other  such  features 
which  have  no  direct  relation  whatever  to  the  character  of  the 
load. 

Therefore  as  load  factor  is  understood,  in  accordance  with 
the  definition  in  the  Standardization  Rules  of  the  A.  I.  E.  E., 
the  ratio  of  the  average  load  to  the  maximum  load;  any 
excess  of  die  station  capacity  beyond  the  maximum  load  is 
power  which  has  not  yet  been  sold,  but  which  is  still  available 
for  the  market,  or  which  is  held  in  reserve  for  emergencies,  is 
not  charged  against  the  load  factor. 

The  cost  of  electric  power  essentially  depends  on  the  load 
factor.  The  higher  the  load  factor,  the  less  is  -the  cost  of  the 
power,  and  a  low  load  factor  means  an  abnormally  high  cost 


54 


GENERAL  LECTURES 


per  kilowatt.  This  is  the  case  in  steam  power,  and  to  a  still 
greater  extent  in  water  power. 

For  the  economical  operation  of  a  system,  it  therefore  is 
of  greatest  importance  to  secure  as  high  a  load  factor  as 
possible,  and  consequently,  the  cost — and  depending  thereon 
the  price — of  electric  power  for  different  uses  must  be  different 
if  the  load  factors  are  different,  and  the  higher  the  cost,  the 
lower  the  load  factor. 

Electrochemical  work  gives  the  highest  load  factor, 
frequently  some  90%,  while  a  lighting  system  shows  the 
poorest  load  factor — in  an  alternating  current  system  without 
motor  load  occasionally  it  is  as  low  as  10  to  20%. 

Denning  the  load  factor  as  the  ratio  of  the  average  to 
the  maximum  load,  it  is  necessary  to  state  over  how  long  a 
time  the  average  is  extended;  that  is,  whether  daily,  monthly 
or  yearly  load  factor. 


SJL 


t-N- 


10 


± 


For  instance,  Fig.  14  shows  an  approximate  load  curve 
of  a  lighting  circuit  during  a  summer  day :  practically  no  load 
except  for  a  short  time  during  the  evening,  where  a  high  peak 


LOAD  FACTOR  AND  COST  OF  POWER        55 


is  reached.    The  ratio  of  the  average  load  to  the  maximum 
load  during  this  day,  or  the  daily  load  factor,  is  22.8%. 

Fig.  15  shows  an  approximate  lighting  load  curve  for  a 
winter  day :  a  small  maximum  in  the  morning,  and  a  very  high 
evening  maximum,  of  far  greater  width  than  the  summer  day 
curve,  giving  a  daily  load  factor  of  34.5^. 


Lot 


During  the  year,  the  daily  load  curve  varies  between  the 
extremes  represented  by  Figs.  14  and  15,  and  the  average 
annual  load  is  therefore  about  midway  between  the  average 
load  of  a  summer  day  and  that  of  a  winter  day.  The  maximum 
yearly  load,  however,  is  the  maximum  load  during  the  winter 


56  GENERAL  LECTURES 

day;  and  the  ratio  of  average  yearly  load  to  maximum  yearly 
load,  or  the  yearly  load  factor  of  the  lighting  system,  therefore 
is  far  lower  than  the  daily  load  factor :  if  we  consider  the  aver- 
age yearly  load  as  the  average  between  14  and  15,  the  yearly 
load  factor  is  only  23.6%. 

One  of  the  greatest  disadvantages  of  lighting  distri- 
bution therefore  is  the  low  yearly  load  factor,  resulting  from 
the  summer  load  being  so  very  far  below  the  winter  load ;  econ- 
omy of  operation  therefore  makes  an  increase  of  the  summer 
lighting  load  very  desirable.  This  has  lead  to  the  development 
of  spectacular  lighting  during  the  summer  months,  as  repre- 
sented by  the  various  Luna  Parks,  Dreamlands,  etc. 


The  load  curve  of  a  factory  motor  load  is  about  the 
shape  shown  in  Fig.  16:  fairly  constant  from  the  opening 
of  the  factories  in  the  morning  to  their  closing  in  the  evening, 
with  perhaps  a  drop  of  short  duration  during  the  noon  hour, 
and  a  low  extension  in  the  evening,  representing  overtime 
work.  It  gives  a  daily  load  factor  of  49.5%. 


-  LOAD  FACTOR  AND  COST  OF  POWER        57 

This  load  curve,  superimposed  upon  the  summer  lighting 
curves,  does  not  appreciably  increase  the  maximum,  but  very 
greatly  increases  the  average  load,  as  shown  by  the  dotted 
curve  in  Fig.  14;  and  so  improves  the  load  factor,  to  65.4% — 
thereby  greatly  reducing  the  cost  of  the  power  to  the  station,  in 
this  way  showing  the  great  importance  of  securing  a  large 
motor  load.  During  the  winter  months,  however,  the  motor 
load  overlaps  the  lighting  maximum,  as  shown  by  the  dotted 
curve  in  Fig.  15.  This  increases  the  maximum,  and  thereby 
increases  the  load  factor  less,  only  to  41.7%.  This  is  not  so 
serious  in  the  direct  current  system  with  storage  battery 
reserve,  as  the  overlap  extends  only  for  a  short  time,  the 
overload  being  taken  care  of  by  storage  batteries  or  by 
the  overload  capacity  of  generators  and  steam  boilers;  but 
where  it  is  feasible,  it  is  a  great  advantage  if  the  users  of 
motors  can  be  induced  to  shut  them  down  in  winter  with 
beginning  darkness. 

It  follows  herefrom,  that  additional  load  on  the  station 
during  the  peak  of  the  load  curve  is  very  expensive,  since  it 
increases  the  fixed  cost  A  and  C,  while  additional  load  during 
the  periods  of  light  station  load,  only  increases  the  proportional 
cost  B;  it  therefore  is  desirable  to  discriminate  against 
peak  loads  in  favor  of  day  loads  and  night  loads.  For 
this  purpose,  two-rate  meters  have  been  developed,  that  is, 
meters  which  charge  a  higher  price  for  power  consumed  dur- 
ing the  peak  of  the  load  curve,  than  for  power  consumed  dur- 
ing the  light  station  loads.  To  even  out  load  curves,  and  cut 
down  the  peak  load,  maximum  demand  meters  have  been 
developed,  that  is,  meters  which  charge  for  power  somewhat 
in  proportion  to  the  load  factor  of  the  circuit  controlled  by 
the  meter.  Where  the  circuit  is  a  lighting  circuit,  and  the 
maximum  demand  therefore  coincides  with  the  station  peak, 


GENERAL  LECTURES 


this  is  effective,  but  on  other  classes  of  load  the  maximum  de- 
mand meters  may  discriminate  against  the  station.  For  in- 
stance, a  motor  load  giving  a  high  maximum  during  some  part 
of  the  day,  and  no  load  during  the  station  peak,  would  be  pref- 
erable to  the  station  to  a  uniform  load  throughout  the  day, 
including  the  station  peak,  while  the  maximum  demand  meter 
would  discriminate  against  the  former. 

By  a  careful  development  of  summer  lighting  loads  and 
motor  day  loads,  the  load  factors  of  direct  current  distribution 
systems  have  been  raised  to  very  high  values,  50  to  60% ;  but 
in  the  average  alternating  current  system,  the  failure  of 
developing  a  motor  load  frequently  results  in  very  unsatisfac- 
tory yearly  load  factors. 


4 


I 


1 


CufiV 


The  load  curve  of  a  railway  circuit  is  about  the  shape  of 
that  shown  in  Fig.  17 :  a  fairly  steady  load  during  the  day,  with 
a  morning  peak  and  an  evening  peak,  occasionally  a  smaller 
noon  peak  and  a  small  second  peak  later  in  the  evening,  then 
tapering  down  to  a  low  value  during  the  night.  The  average 


LOAD  FACTOR  AND  COST  OF  POWER        59 

load  factor  usually  is  far  higher  than  in  a  lighting  circuit,  in 
Fig.  17:54.3%- 

In  defining  the  load  factor,  it  is  necessary  to  state  not 
only  the  time  over  which  the  load  is  to  be  averaged,  as  a  day,  or 
a  year,  but  also  the  length  of  time  which  the  maximum  load 
must  last,  must  be  counted.  For  instance,  a  short  circuit  of  a 
large  motor  during  peak  load,  which  is  opened  by  the  blowing 
of  the  fuses,  may  momentarily  carry  the  load  far  beyond  the 
station  peak  without  being  objectional.  The  minimum  dura- 
tion of  maximum  load,  which  is  chosen  in  determining  the 
load  factor,  is  that  which  is  permissible  without  being  ob- 
jectionable for  the  purpose  for  which  the  power  is  distributed. 
Thus  in  a  lighting  system,  where  voltage  regulation  is  of  fore- 
most importance,  minutes  may  be  chosen,  and  maximum  load 
may  be  defined  as  the  average  load  during  that  minute  during 
which  the  load  is  a  maximum;  while  in  a  railway  system,  a 
half -hour  may  be  used  as  a  duration  of  maximum  load,  as  a 
railway  system  is  not  so  much  affected  by  a  drop  of  voltage  due 
to  overload,  and  an  overload  of  less  than  half  an  hour  may  be 
carried  by  the  overload  capacity  of  the  generators  and  the  heat 
storage  of  the  steam  boilers ;  so  that  a  peak  load  requires  seri- 
ous consideration  only  when  it  exceeds  half  an  hour. 


FIFTH  LECTURE 


T 


LONG  DISTANCE  TRANSMISSION 

HREE-PHASE  is  used  altogether  for  long  distance 
transmission.  Two-phase  is  not  used  any  more,  and 
direct  current  is  being  proposed,  having  been  used 
abroad  in  a  few  cases;  but  due  to  the  difficulty  of  generation 
and  utilization,  is  not  probable  that  it  will  find  any  extended 
use,  so  that  it  does  not  need  to  be  considered. 

FREQUENCY 

The  frequency  depends  to  a  great  extent  on  the  character 
of  the  load,  that  is,  whether  the  power  is  used  for  alternating 
current  distribution — 60  cycles — or  for  conversion  to  direct 
current — 25  cycles.  Pvor  .the  transmission  line,  25  cycles  has  the 
advantage  that  the  charging  current  is  less  and  the  inductive 
drop  is  less,  because  charging  current  and  inductance  voltage 
are  proportional  to  the  frequency. 

VOLTAGE 

n,ooo  to  I3>2OO  volts  and  more  recently,  even  22,000 
volts  is  most  common  for  shorter  distances,  as  10  to  20  miles, 
since  this  is  about  the  highest  voltage  for  which  generators  can 
be  built;  its  use  therefore  saves  the  step-up  transformers,  that 
is,  the  generator  feeds  directly  into  the  line  and  to  the  step- 
down  transformers  for  the  regular  load. 

The  next  step  is  30,000  volts;  that  is,  33,000  volts  at  the 
generator,  30,000  at  the  receiving  end  of  the  line.  No  inter- 
mediate voltages  between  this  and  the  voltage  for  which 
generators  can  be  wound  is  used,  as  30,000  volts  does  not 
yet  offer  any  insulator  troubles ;  but  line  insulators  can  be  built 
at  moderate  cost  for  this  voltage,  and  as  step-up  transformers 


64  GENERAL  LECTURES 

have  to  be  used,  it  is  not  worth  while  to  consider  any  lower 
voltage  than  33,000  volts.  This  voltage  transmits  economically 
up  to  distances  of  50  to  60  miles. 

40,000  to  44,000  volts  is  the  next  step ;  it  is  used  for  high 
power  transmission  lines  of  greater  distance,  where  reliability 
of  operation  is  of  importance  and  the  use  of  a  conservative 
voltage  therefore  preferable  to  the  attempt  at  economizing  by 
the  use  of  extra  high  voltages. 

A  number  of  60,000  volt  systems  are  in  more  or  less 
successful  operation,  and  systems  of  80,000  to  110,000  volts 
are  in  construction  and  a  few  in  operation.  Where  the  dis- 
tances are  very  great,  power  valuable,  and  continuity  of  ser- 
vice not  of  such  foremost  importance,  such  voltages  are  justi- 
fied in  the  present  state  of  the  art. 

In  such  very  high  voltage  systems,  the  transformers  are 
occasionally  wound  so  that  they  can  be  connected  for  half 
voltage,  for  operating  the  line  at  half  voltage,  until  the  load 
has  sufficiently  increased  to  require  full  voltage;  or  the 
transformers  are  built  for  star  or  Y  connection  at  full 
voltage,  and  at  first  operated  in  ring  or  delta  connection, 

i 

at =  57%  of  full  voltage. 

V3 

The  cost  of  a  long  distance  transmission  line  depends  on 
the  voltage  used. 

The  cost  of  line  conductors  decreases  with  the  square  of 
the  voltage. 

At  twice  the  voltage,  twice  the  line  drop  can  be  allowed 
with  the  same  loss;  at  twice  the  voltage  the  current  is  only 
half  for  the  same  power,  and  twice  the  drop  with  half  the 
current  gives  four  times  the  resistance,  that  is,  one-quarter 
the  conductor  section  and  cost. 


LONG  DISTANCE  TRANSMISSION  65 

The  cost  of  line  insulators  increases  with  increase  of 
voltage.  The  cost  of  pole  line  increases  with  increase  of 
voltage,  since  greater  distance  between  the  conductors  is 
necessary  and  so  longer  poles,  longer  cross  arms,  and  heavier 
construction,  and  not  so  many  circuits  can  be  carried  on  the 
same  pole  line. 

The  lower  the  voltage,  the  greater  in  general  is  the  reli- 
ability of  operation,  since  a  larger  margin  of  safety  can  be 
allowed. 

Since  a  part  of  the  cost  of  the  transmission  line  decreases, 
another  part  increases  with  the  voltage,  a  certain  voltage  will 
be  most  economical. 

Lower  voltage  increases  the  cost  of  the  conductor,  higher 
voltage  increases  the  cost  of  insulators  and  line  construction, 
and  decreases  the  reliability. 

The  most  economical  voltage  of  a  transmission  line  varies 
with  the  cost  of  copper.  When  copper  is  very  high,  higher 
voltages  are  more  economical  than  when  copper  is  low.  The 
same  applies  to  aluminum,  since  the  price  of  aluminum  has 
been  varied  with  that  of  copper. 

Aluminum  generally  is  used  as  stranded  conductor.  In 
the  early  days  single  wire  gave  much  trouble  by  flaws  in  the 
wire.  Aluminum  expands  more  than  copper  with  temperature 
changes,  and  so  when  installing  the  line  in  summer,  a  greater 
sag  must  be  allowed  than  with  copper,  otherwise  it  stretches 
so  -tight  in  winter  that  it  may  tear  apart.  Aluminum  also  is 
more  difficult  to  join  together,  since  it  cannot  be  welded. 

For  the  same  conductivity  an  aluminum  line  has  about 
twice  the  size,  but  one-half  of  the  weight  of  a  copper  conductor, 
and  costs  10%  less;  but  copper  has  a  permanent  value,  while 
the  price  of  aluminum  may  sometime  drop  altogether,  as  the 
metal  has  no  intrinsic  value,  being  one  of  >the  most  common 


66 


GENERAL  LECTURES 


constituents  of  the  surface  of  the  earth,  and  its  cost  is  merely 
that  of  its  separation  or  reduction. 

LOSSES  IN  LINE  DUE  TO  HIGH  VOLTAGE 

The  loss  in  the  line  by  brush  discharge  or  corona  effect 
is  nothing  up  to  a  certain  voltage,  but  at  a  certain  voltage  it 
begins  and  very  rapidly  increases. 

The  voltage  at  which  a  loss  by  corona  effect  begins  is 
where  the  air  at  the  surface  of  the  conductor  breaks  down, 
becomes  conducting  and  thus  luminous.  This  occurs  at  a 
potential  gradient  of  100,000  to  120,000  volts  per  inch. 

The  potential  gradient  is  highest  at  the  surface  of  the 
conductor. 


In  Fig.  18  let 

R  =  radius  of  conductor. 
2  d  =  distance  between  conductor  centres. 
At  a  point  x  from  the  centre  O  the  potential  is : 

C  C  2  C  X 

Tf    __________   __________    

d  -  -  x       d  +  x        d2  —  x2 
f  or :  x  =  d  —  R 

that  is,  at  the  conductor  surface,  it  is : 
i  =  e 


LONG  DISTANCE  TRANSMISSION  67 

Substituting  this  in  the  equation,  gives  : 

c 

e  =  — 
R 

hence  : 

c  =  eR 

therefore  the  potential  at  point  x  is  : 

2    R   X 

f  =  -  e 
d2  —  x2 

and  the  potential  gradient: 

d  f          2  R(d2  +  x2) 


d  x  (d2  —  x2)2 

e 

hence  for  :  x  =  d  —  R  or  the  conductor  surface  :  g0  =  — 

R 

If  this  potential  gradient  becomes  greater  than  the  break- 
down strength  of  air,  or  100,000  volts  per  inch,  corona  effects 
and  energy  losses  take  place: 
e 

—  =  100,000 

R 

gives: 

e  =  100,000  R  or  E  =  100,000  D,  as  the  voltage  where 
the  corona  begins,  and  : 

e  E 

R  =  -  or  D  =  -   is  the  smallest  radius 
100,000  100,000 

which  can  be  used,  at  voltage  E,  where  D  is  the  conductor 
diameter  =  2  R,  and  E  is  the  voltage  between  the  conductor  = 
2  e. 

For  instance,  wire  No.  oooo 

D  =  .46"  ;  corona  effects  begin  at  the  voltage  E  =  100,000 
D  =  46,000. 


68  GENERAL  LECTURES 

For  100,000  volts  (the  smallest  diameter  for  which  no 
corona  effects  occur  is : 

E 

D  = =  i" 

100,000 

In  high  potential  transformers  in  the  coils  no  corona 
effects  may  occur,  because  the  diameter  of  the  coil  or  the  thick- 
ness is  large  enough,  but  the  leads  connecting  the  coils  with 
each  other  and  with  the  outside,  if  not  chosen  very  large  in 
diameter,  may  give  corona  effects  and  so  break  down. 

In  a  line  or  transformer,  if  one  side  is  grounded,  the  other 
side  has  full  voltage  against  ground,  and  so  may  give  corona 
effects  and  break  down ;  while  if  not  grounded,  both  sides  have 
half  voltage  against  ground  and  so  give  no  corona  effect.  In 
the  first  case,  the  line  or  transformer  so  may  break  down, 
although  the  potential  differences  between  the  terminals  are 
no  greater  than  in  the  second  case. 

For  instance,  in  a  100,000  volt  transformer  or  line,  from 
each  terminal  to  ground  are  50,000  volts,  and  if  the  conductor 
diameter  is  V2",  no  corona  effects  occur.  If  now  one  terminal 
is  grounded,  the  other  terminal  has  100,000  volts  to  ground 
and  so  at  Y2"  diameter  gives  corona  effects,  thait  is,  glow  and 
streamers  which  may  destroy  the  insulating  material  or 
produce  high  frequency  oscillations. 

At  very  high  voltages  it  is  therefore  necessary  to  have 
the  system  statically  balanced  or  symmetrical,  that  is,  have  the 
same  potential  differences  from  all  the  conductors  to  the 
ground. 

Any  electric  circuit,  and  so  also  the  transmission  line, 
contains  inductance  and  capacity,  and  therefore  stores  energy 
as  electromagnetic  energy  in  the  magnetic  field  due  to  the  cur- 
rent, and  as  electrostatic  energy,  or  electrostatic  charge,  due  to 
the  voltage. 


LONG  DISTANCE  TRANSMISSION  69 

If: 

e  =  voltage,   C  =  capacity, 
i  =  current,    L  =  inductance, 
the  electrostatic  energy  is : 
e2C 


2 

and  the  electromagnetic  energy : 
i2L 


In  a  high  potential  transmission  line  both  energies  are 
of  about  the  same  magnitude,  and  the  energy  can  therefore  see- 
saw between  the  two  forms  and  thereby  produce  oscillations 
and  surges  resulting  in  the  production  of  high  voltages,  which 
are  not  liable  to  occur  in  circuits  in  which  one  of  the  forms  of 
stored  energy  is  small  compared  with  the  other. 

In  distribution  systems  up  to  2200  volts  and  even  some- 
what higher,  the  electrostatic  energy  is  still  negligible  and 
only  the  electromagnetic  energy  appreciable. 

In  static  machines  the  electrostatic  energy  is  appreciable, 
but  the  electromagnetic  energy  negligible. 

LINES  AND  TRANSFORMERS 

At  voltages  above  2$,ooo  step-up  and  step-down  trans- 
formers are  always  used,  which  are  therefore  a  part  of  the  high 
potential  circuit. 

Three-phase  is  always  used  in  the  transmission  line. 

Some  of  the  available  transformer  connections  are  given 
in  Figs.  19  and  20. 

Grounding  the  neutral  of  the  system  has  the  advantage  of 
maintaining  static  balance  and  so  avoiding  oscillations  and 
disturbances  in  case  of  an  accidental  static  unbalancing,  as  for 


7° 


GENERAL  LECTURES 


aeir/1-Y 


instance,  the  grounding  of  one  line.  It  has  the  disadvantage 
that  a  ground  on  one  circuit  is  a  short  circuit  and  so  shuts 
down  the  circuit. 


LONG  DISTANCE  TRANSMISSION  71 

In  connections  i,  4  and  6  no  neutral  is  available  for 
grounding  and  so  three  separate  transformers  have  to  be 
installed  in  Y  connection  for  getting  the  neutral. 

In  connections  2  and  3  the  neutral  can  be  brought  out 
from  the  transformer  neutral. 


In  the  T  connection  5  and  7,  the  neutral  is  brought  out 
from  a  point  at  one-third  of  the  teaser  transformer  winding. 

Assuming  the  line  properly  installed  and  insulated,  break- 
downs may  occur,  either  from  mechanical  accidents  or  by  high 
voltages  appearing  in  the  line. 


7i  GENERAL  LECTURES 

HIGH  VOLTAGE  DISTURBANCES  IN 
TRANSMISSION  LINES 

These  may  be: 

A.  Of  fundamental  frequency,  that  is,  the  same  frequency 
as  the  alternating  current  machine  circuit. 

B.  Some  higher  harmonic  of  the  generator  wave,  that  is, 
some  odd  multiple  of  the  generator  frequency. 

C.  Of  frequencies  entirely  independent  of  the  generator, 
or  of  a  frequency  which  originates  in  the  circuit,  that  is,  high 
frequency  oscillations  as  arcing  grounds,  etc. 

If  a  capacity  is  in  series  with  an  inductance,  as  the  line 
capacity  and  the  line  inductance,  the  capacity  reactance  and 
the  inductive  reactance  are  opposed  to  each  other ;  if  they  hap- 
pened to  be  equal  they  would  neutralize  each  other,  the  current 
would  depend  on  the  resistance  only  and  therefore  be  very 
large,  and  with  this  very  large  current  passing  through  the 
inductance  and  capacity,  the  voltage  at  the  inductance  and  at 
the  capacity  would  be  very  high. 

For  instance,  if  we  have  20,000  volts  supplied  to  a  circuit 

having  a  resistance  of  10  ohms  and  a  capacity  reactance  of 

1000    ohms,    then    the    total    impedance    of    the    circuit    is 

io2  +   iooo2  =    1000    and    the    current    in    the    circuit 

20,000 

=  20  amperes. 

iooo 

If  now  in  addition  to  the  io  ohms  resistance  and  iooo 
ohms  capacity  reactance,  the  circuit  contains  iooo  ohms 
inductive  reactance,  the  total  reactance  of  the  circuit  is 
iooo  —  iooo  =  o  ohms,  and  the  impedance  is  the  same  as 

e        e 

the  resistance,  or  io  ohms.    The  current  therefore  —  =  —  = 

z         r 


LONG  DISTANCE  TRANSMISSION  73 

2000  amperes,  and  the  voltage  at  the  capacity  therefore  is: 
capacity  reactance  times  amperes  =  2,000,000  volts,  and  the 
same  voltage  exists  at  the  inductive  reactance. 

These  voltages  are  far  beyond  destruction.  That  is,  if 
in  a  circuit  of  low  resistance  and  high  capacity  reactance,  a 
high  inductive  reactance  is  put  in  series  with  the  capacity 
reactance,  excessive  voltages  are  produced. 

In  a  .transmission  line  the  capacity  of  the  line  consumes 
for  instance  10%  of  full  load  current;  that  is,  full  load  voltage 
sends  only  10%  of  full  load  current  through  the  capacity.  To 
send  full  load  current  through  the  capacity  so  would  require  10 
times  full  load  voltage. 

With  a  line  reactance  of  20%,  20%  or  1/5  of  full  load 
voltage  sends  full  load  current  through  the  inductive  reactance, 
while  10  times  full  load  voltage  is  required  by  the  capacity 
reactance;  the  capacity  reactance  therefore  is  about  50  times 
larger  than  the  inductive  reactance  at  the  generator  frequency 
and  therefore  cannot  build  up  with  it  to  excessive  voltages ;  but 
to  get  resonance  with  the  fundamental  frequency  requires  an 
inductive  reactance  about  50  times  greater  than  the  line 
reactance. 

The  only  reactance  in  the  system  which  is  large  enough  to 
build  up  with  the  capacity  reactance  is  the  open  circuit 
reactance  of  the  transformers.  This  is  of  about  the  same  size 
as  the  capacity  reactance,  since  a  transformer  at  open  circuit 
and  full  voltage  takes  about  10%  of  full  load  current,  and  the 
capacity  reactance  also  takes  about  10%  of  full  load  current. 

If  therefore  a  high  potential  coil  of  a  transformer  at  open 
secondary  circuit  is  connected  in  series  with  a  transmission 
line,  destructive  voltages  may  be  produced,  by  the  reactance 
of  the  transformer  building  up  with  the  line  capacity. 
In  those  transformer  connections  in  which  several  high 


74 


GENERAL  LECTURES 


potential  coils  of  different  transformers  are  connected  between 
the  transmission  wires,  this  may  occur  if  the  low  tension  coil 
of  one  of  the  transformers  accidentally  opens  and  the  high 
potential  coil  of  this  transformer  then  acts  as  inductive  react- 
ance in  series  with  the  line  capacity  in  the  circuit  of  the  other 
transformer. 

1 


This  may  occur  for  insitance  in  transformer  connection  2, 
Fig.  19,  if  as  shown  in  Fig.  21,  the  low  tension  coil  c  opens. 
Then  the  high  tension  coil  C  is  an  inductive  reactance  in  series 


c      />• 


1        -L 

*          .             2 

<r     & 

9 

with  the  line  capacity  from  3  to  i,  energized  by  transformer 
A;  and  C  is  a  high  inductive  reactance  in  series  with  the  line 
capacity  from  3  to  2  in  a  circuit  of  voltage  B.  That  is,  from 
3  to  i  and  from  3  to  2  excessive  voltages  are  produced.  So 
also  in  T  connection,  Fig.  22,  if  for  instance  the  low  tension 
coil  a  opens,  the  corresponding  high  tension  coil  A  is  a  high 
inductive  reactance  in  series  with  the  line  capacities  in  a  circuit 


LONG  DISTANCE  TRANSMISSION  75 

of  the  voltages  of  the  two  halves,  B  and  C,  of 'the  other  trans- 
former, and  excessive  voltages  therefore  appear  from  i  to  2 
and  from  i  to  3. 

This  danger  of  excessive  voltages  by  the  accidental  open- 
ing of  a  transformer  low  tension  coil  does  not  exist  in  delta 
connection,  since  in  this  always  only  one  transformer  connects 
from  line  to  line.  It  is  greatly  reduced  since  the  use  of  triple 
pole  switches  became  general;  and  is  very  much  less  where 
several  sets  of  transformers  are  used  in  multiple,  since  even  if 
in  one  set  a  low  tension  coil  opens,  the  other  sets  maintain  the 
voltage  triangle. 

Especially  dangerous  in  this  respect  therefore  is  the  L 
connection  No.  6;  since  in  this  case,  when  using  two  trans- 
formers in  open  delta,  for  smaller  systems  only  one  set  is 
installed  and  an  accident  to  one  of  the  -transformers  causes 
excessive  voltages  between  its  line  and  the  two  other  lines. 

The  open  circuit  reactance  of  the  transformer  is  the  only 
reactance  high  enough  to  give  destructive  voltages  at  gener- 
ator frequency,  and  in  high  potential  disturbances,  the  trans- 
former connections  should  first  be  carefully  investigated  to 
see  whether  this  has  occurred. 


SIXTH  LECTURE 


HIGHER  HARMONICS  OF  THE 
GENERATOR  WAVE 

HE  open  circuit  reactance  of  the  transformer  is  the  only 
reactance  high  enough  to  give  resonance  with  the  line 
capacity  at  fundamental  frequency. 

All  other  reactances  are  too  low  for  this. 

Since,  however,  the  inductive  reactance  increases  and  the 
capacity  reactance  decreases  proportionally  to  the  frequency, 
the  two  reactances  come  nearer  together  for  higher  frequency ; 
that  is,  for  the  higher  harmonics  of  the  generator  wave,  and 
for  some  of  the  higher  harmonics  of  the  generator  wave 
resonance  rise  of  voltage  so  may  occur  between  the  line 
capacity  and  the  circuit  inductance. 

The  origin  and  existence  of  higher  harmonics  therefore 
bears  investigation  in  transformers,  transmission  lines  and 
cable  systems. 

ORIGIN  OF  HIGHER  HARMONICS 

Higher  harmonics  may  originate  in  synchronous  machines, 
as  generators,  synchronous  motors  and  converters,  and  in 
transformers. 

These  two  classes  of  higher  harmonics  are  very  different. 
The  former  have  constant  potential  character;  the  latter,  con- 
stant current  character;  their  cure  and  prevention  there- 
fore must  be  different,  and  the  method  of  elimination  of  one 
may  be  very  harmful  with  the  other  type  of  harmonics.  For 
instance,  the  voltage  produced  by  a  constant  current  harmonic 
as  coming  from  a  transformer  is  eliminated  by  short  circuit. 
Short  circuiting  a  generator  harmonic,  however,  gives  large 


8o  GENERAL  LECTURES 

short  circuit  currents,  due  to  the  constant  potential  character, 
and  is  therefore  dangerous. 

HIGHER  HARMONICS  OF  SYNCHRONOUS 
MACHINES 

In  synchronous  machines,  as  alternating  current  genera- 
tors, the  higher  harmonics  are: 

AT  No  LOAD 

ist.  The  distribution  of  magnetism  in  the  air  gap 
depends  on  the  shape  of  the  field  poles ;  it  is  not  a  sine  wave ; 
neither  is  the  e.  m.  f .  induced  by  it  in  an  armature  a  sine  wave. 

Since  there  are  a  number  of  conductors  in  series  on  the 
armature,  the  voltage  wave  is  more  evened  out  than  that  of  a 
single  conductor ;  but  still  it  is  not  a  sine  wave,  that  is,  contains 
harmonics  of  which  the  third  is  the  lowest. 

2nd.  The  change  of  magnetic  flux  by  the  passage  of  open 
armature  slots  over  the  field  pole  produces  harmonics  of 
e.  m.  f . ;  that  is,  when  a  large  open  armature  slot  stands  in  front 
of  the  field  pole,  the  magnetic  reluctance  is  high ;  the  magnetism 
is  lower  than  when  no  slot  is  in  front  of  the  field  pole ;  that  is, 
by  the  passage  of  the  armature  slots  the  field  magnetism  pul- 
sates, the  more  so  the  larger  the  slots  and  the  fewer  they  are. 

If  there  are  n  slots  per  pole,  this  produces  the  two  har- 
monics 2n  —  i  and  2n  +  I. 

AT  LOAD 

3rd.  The  armature  reaction  of  a  single-phase  machine 
pulsates  between  zero  at  zero  current  and  a  maximum  at  maxi- 
mum current. 

The  resultant  armature  reaction  of  a  polyphase  machine 
is  constant,  but  locally  there  is  a  pulsation  making  as  many 
cycles  per  pole  as  there  are  phases. 


HARMONICS  OF  GENERATOR  WAVE  8 1 

Since  the  field  magnetism  under  load  is  due  to  the  com- 
bination of  field  excitation  and  armature  reaction,  the  pulsa- 
tion of  armature  reaction  therefore  causes  a  pulsation  of  field 
magnetism,  and  thereby  higher  harmonics  of  the  e.  m.  f .  wave. 

If  m  =  number  of  phases,  the  higher  harmonics :  2m  —  I 
and  2m  +  i  are  produced. 

4th.  The  terminal  voltage  under  load  is  the  resultant  of 
the  induced  e.  m.  f.  and  the  e.  m.  f .  consumed  by  the  reactance 
of  the  armature  circuit ;  that  is,  the  reactance  produced  by  the 
magnetic  flux  produced  by  the  armature  current  in  the  arma- 
ture iron.  This  armature  reactance  is  not  constant,  but  peri- 
odically varies,  more  or  less,  with  double  frequency;  that  is, 
when  the  armature  coil  is  in  front  of  the  field  pole  its  magnetic 
circuit  is  different  than  when  it  is  between  the  field  poles,  and 
the  reactance  therefore  is  different. 

This  pulsation  of  armature  reactance  produces  the  third 
harmonic,  since  it  is  of  double  frequency. 

The  most  common  and  prominent  harmonic  so  is  the  third 
harmonic  in  a  synchronous  machine. 

These  harmonics  of  synchronous  machines  are  induced 
e.  m.  f's,  that  is,  constant  potential  or  approximately  so. 

HIGHER  HARMONICS  OF  TRANSFORMERS 

In  a  transformer  the  wave  of  e.  m.  f.  depends  on  that  of 
the  magnetism  and  vice  versa.  That  is,  with  a  sine  wave  of 
e.  m.  f.,  the  magnetism  must  also  be  a  sine  wave,  and  if  the 
magnetism  is  not  a  sine  wave,  but  contains  higher  harmonics, 
the  e.  m.  f.  is  not  a  sine  wave,  but  contains  the  harmonics 
induced  by  the  harmonics  of  magnetism. 

The  exciting  current  of  the  transformer  depends  on  the 
magnetism  by  the  hysteresis  cycle;  if  the  magnetism  is  a  sine 
wave,  the  exciting  current  therefore  cannot  be  a  sine  wave,  but 


82 


GENERAL  LECTURES 


must  contain  higher  harmonics- — mainly  the  third  harmonic, 
which  reaches  20  to  30%  of  the  fundamental,  or  even  more  at 
saturation. 


HARMONICS  OF  GENERATOR  WAVE  83 

In  a  transformer,  e.  m.  f.  and  exciting  current  therefore 
cannot  both  be  sine  waves,  but  a  sine  wave  of  e.  m.  f .  requires 
an  exciting  current  containing  a  third  harmonic;  and  a  sine 
wave  of  exciting  current  in  a  transformer  or  reactive  coil  thus 
produces  a  third  harmonic  of  e.  m.  f. 

If  therefore  in  a  transformer  the  third  harmonic  is  sup- 
pressed, and  if  this  third  harmonic  should  have  been  20%  of 
the  fundamental,  then  its  suppression  produces  a  third  har- 
monic of  magnetism  of  20%  in  the  opposite  direction. 
A  third  harmonic  of  magnetism,  however,  of  20%,  induces  a 
third  harmonic  of  e.  m.  f .  of  3  x  20  =  60%  ;  the  e.  m.  f.  being 
proportional  to  magnetism  and  frequency. 

The  third  harmonic  of  exciting  current  is  positive  at  the 
maximum  of  magnetism,  and  the  third  harmonic  of  magnetism 
is  negative  at  the  maximum,  hence  is  zero  and  rising  at  the  zero 
of  the  magnetism ;  and  at  this  moment  the  e.  m.  f .  induced  by 
the  third  harmonic  and  by  the  fundamental  therefore  are  both 
maxima  and  in  the  same  direction,  that  is,  add.  The  suppres- 
sion of  the  third  harmonic  of  exciting  current  thus  produces  a 
very  high  third  harmonic  of  e.  m.  f.,  which  greatly  increases 
the  maximum  e.  m.  f . ;  that  is,  the  e.  m.  f .  wave  is  very  low  for 
a  large  part  of  the  cycle  and  then  rises  to  a  very  high  peak,  as 
shown  by  Fig.  23 ;  and  the  maximum  e.  m.  f .  may  exceed  that 
of  a  sine  wave  by  50%  and  more,  thus  giving  high  insulation 
stress  and  the  possibility  of  resonance  voltages. 

EFFECTS  OF  HIGHER  HARMONICS 

In  a  three-phase  system  the  three  phases  are  120°  apart, 
and  their  third  harmonics  are  3  x  120°  =  360°  apart,  that  is,  in 
phase  with  each,  and  for  the  third  harmonic  the  three-phase 
system  therefore  is  a  single-phase  system. 


84  GENERAL  LECTURES 

In  a  balanced  three-phase  system,  the  third  harmonics  can 
not  exist  in  the  voltages  between  the  lines  and  in  the  line 
currents,  if  there  is  no  return  over  the  neutral.  The  three 
voltages  between  lines,  from  i  to  2,  2  to  3,  and  3  to  i,  must  add 
up  to  zero;  but  since  the  third  harmonics  would  be  in  phase 
with  each  other,  they  would  not  add  up  to  zero,  therefore  they 
cannot  exist.  The  three  currents,  if  there  is  no  return  over  the 
neutral  or  the  ground,  must  add  up  to  zero;  and  since  their 
third  harmonics  must  be  in  phase  with  each  other,  they  must 
be  absent.  In  a  balanced  three-phase  system,  third  harmonics 
can  exist  only  in  the  voltage  from  line  to  neutral  or  Y  voltage, 
in  the  current  from  line  to  line  or  delta  current,  and  in  the 
line  current  only  if  there  is  a  neutral  return  or  ground  return 
to  the  generator  neutral  or  transformer  neutral. 

In  a  three-phase  generator,  if  the  e.  m.  f .  of  one  phase  con- 
tains a  third  harmonic,  as  is  usually  the  case,  then  by  connect- 
ing the  three  phases  in  delta  connection,  the  third  harmonics 
of  the  generator  e.  m.  f.'s  are  short  circuited  and  so  produce  a 
triple  frequency  current  circulating  in  the  generator  delta. 
This  triple  frequency  circulating  current  can  be  measured  by 
connecting  an  ammeter  in  one  corner  of  the  generator  delta, 
and  the  sum  of  voltages  of  the  three  third  harmonics  can  be 
measured  by  putting  a  voltmeter  in  a  corner  of  the  generator 
delta.  This  local  current  in  the  generator  winding  is  the  triple 
frequency  voltage  divided  by  the  generator  impedance  (the 
stationary  impedance,  at  triple  frequency,  but  not  the  syn- 
chronous impedance,  since  the  latter  includes  armature  reac- 
tion). In  generators  of  low  impedance  or  close  regulation, 
as  turbine  alternators,  this  local  current  may  be  far  more  than 
full  load  current ;  delta  connection  of  generator  windings  there- 
fore is  unsafe.  As  a  result,  generator  windings  are  almost 
always  connected  in  Y.  Even  with  delta  connection  of  gener- 


HARMONICS  OF  GENERATOR  WAVE  85 

ator  windings  no  triple  frequency  appears  at  the  terminals, 
since  its  voltage  disappears  by  short  circuit. 

If  the  generator  winding  is  connected  in  Y,  the  triple 
frequency  voltages  from  terminal  to  neutral  are  in  phase  with 
each  other;  that  is,  in  a  three-phase  Y  connected  generator,  a 
single-phase  voltage  of  triple  frequency  exists  between  the 
neutral  and  all  'three  terminals,  and  the  neutral  therefore  is  not 
a  true  neutral.  Between  the  lines  no  triple  frequency  voltage 
exists,  since  from  terminal  to  neutral  and  from  neutral  to  the 
other  terminal  the  two  third  harmonics  are  in  opposition  and 
so  neutralize. 

This  third  harmonic  between  generator  neutral  and  line 
must  be  kept  in  mind,  since  when  large  it  may  produce  danger- 
ous voltages  by  resonance  with  the  line  capacity. 

When  the  generator  neutral  is  grounded,  the  potential 
difference  from  line  to  ground  is  not  line  voltage  divided  by 
V3,  that  is,  the  true  Y  voltage  of  the  system ;  but  superimposed 
upon  it  is  this  single-phase  triple  frequency  voltage;  and  the 
voltage  from  line  to  ground,  especially  its  maximum,  may  be 
greatly  increased,  thus  increasing  the  insulation  strain.  For 
this  single-phase  voltage  all  three  lines  go  together,  and  so  may 
cause  static  induction  on  other  circuits,  as  telephone  lines.  A 
circuit  of  this  single-phase  triple  frequency  voltage  then  exists 
from  the  generator  neutral  over  the  inductance  of  all  three 
generator  circuits  in  multiple,  and  over  the  capacity  of  all  three 
lines  to  ground,  back  to  the  generator  neutral ;  that  is,  we  have 
capacity  and  inductance  in  series  in  a  circuit  of  the  triple  har- 
monic, and  if  capacity  and  inductance  are  high  enough,  we 
may  get  a  dangerous  voltage  rise. 

In  this  case  of  grounded  generator  neutral,  if  the  neutral 
of  the  Y  connected  step-down  transformers  is  grounded  also, 
and  the  low  tension  side  of  these  transformers  connected  in  Y, 


86  GENERAL   LECTURES 

the  third  harmonic  of  the  generator  has  no  path;  the  cur- 
rent produced  by  it  would  have  to  return  over  the  open  circuit 
reactance  of  the  step-down  transformer,  and  is  limited  there- 
by to  a  negligible  value. 

If,  however,  the  secondaries  of  the  step-down  trans- 
formers are  connected  in  delta,  so  that  the  third  harmonic  can 
circulate  in  the  secondary  delta,  the  third  harmonic  can  flow 
through  the  transformer  primary  by  inducing  an  opposite  cur- 
rent in  the  secondary;  in  this  case  the  step-down  trans- 
former short  circuits  the  third  harmonic  of  the  generator. 
Grounding  the  primary  neutral  of  step-down  transformers 
with  grounded  generator  neutral  therefore  is  permissible  only 
if  the  transformer  secondaries  are  also  connected  in  Y.  With 
delta  connected  transformer  secondaries,  however,  it  is  not 
safe  to  ground  the  generator  neutral  and  transformer  neutral ; 
since  this  produces  a  triple  frequency  current  in  generator,  line 
and  transformer ;  and  even  if  the  generator  reactance  is  so  high 
that  the  generator  is  not  harmed  by  this  current,  it  may  burn 
out  at  the  transformer,  and  probably  will  do  so  if  the  trans- 
former is  small  compared  with  the  generator. 

This  therefore  is  a  case  where  delta  connection  of  the 
transformer  secondaries  does  not  eliminate  the  trouble  from 
the  third  harmonic,  but  makes  it  worse. 

The  (triple  frequency  voltage  from  line  to  ground  would 
be  eliminated  by  short  circuiting  it  in  this  manner,  by  Y  delta 
connection  of  step-down  transformer  with  grounded  generator 
and  transformer  neutral,  and  static  induction  on  other  circuits 
so  would  disappear;  but  we  get  magnetic  induction  from  the 
three  triple  frequency  single-phase  currents  which  now  flow 
over  the  lines  to  the  ground. 

If  the  generator  neutral  is  not  grounded,  it  is  safe  to 
ground  transformer  neutrals.  With  ungrounded  generator 


HARMONICS  OF  GENERATOR  WAVE  87 

neutral,  a  triple  frequency  voltage  can  be  measured  by  volt- 
meter, which  then  appears  between  generator  neutral  and 
ground;  this  voltage  under  unfavorable  conditions,  may  give 
insulation  strains  in  the  generator  by  resonance  rise;  in  the 
circuit  from  generator  neutral  over  triple  frequency  voltage, 
generator  inductance,  capacity  from  line  to  ground  and  capac- 
ity from  ground  to  generator  winding  in  series. 

In  this  case  the  capacity  is  much  lower  and  the  power 
therefore  much  less,  that  is,  less  danger  exists. 

When  running  two  or  more  three-phase  generators  in 
parallel,  with  grounded  neutrals: 

a.  If  the  generators  have  different  third  harmonics,  these 
harmonics  are  short  circuited  from  neutral  over  generator  to 
the  other  generator  and  back  to  neutral ;  a  triple  frequency 
current  thus  flows  between  the  generators,  that  is,  the  current 
between  the  generators  can  never  be  made  to  disappear. 

That  is,  for  the  third  harmonic,  the  two  generators  are 
two  single-phase  machines  of  different  voltage,  having  the 
neutral  as  one  terminal  and  the  three  three-phase  terminals  as 
the  other  single-phase  terminal. 

b.  With  two  identical  generators  running  in  multiple,  if 
the  excitation  is  identically  the  same,  no  current  flows  between 
the  grounded  neutrals.    If  the  excitation  of  the  two  generators 
is  different,  one  is   over-excited    the   other   is   under-excited 
(that  is,  one  carries  leading,  the  other  lagging  current)  then  a 
triple  frequency  current  flows  between  the  neutrals  of  identical 
generators.     Since  in  parallel  operation  the  terminal  voltages 
are  in  phase,  if  by  difference  of  excitation  the  two  terminal 
voltages  have  a  different  lag  behind  the  induced  e.  m.  f.'s,  the 
third  harmonics,  which  lag  three  times  as  much  as  the  funda- 
mentals, cannot  be  in  phase  in  the  two  machines;  and  thus 
triple  frequency  current  flows  between  the  machines. 


88  GENERAL  LECTURES 

In  machines  of  very  low  reactance  as  turbo-alternators, 
even  small  differences  in  excitation  of  identical  machines  with 
grounded  neutral  may  thus  cause  very  large  neutral  currents. 

In  parallel  operation  of  three-phase  machines  with 
grounded  neutral,  machines  of  different  wave  shapes  frequently 
cannot  be  run  together  at  all  without  excessive  neutral  currents, 
and  the  ground  has  to  be  taken  off  of  one  of  the  machine  types. 

Even  with  identical  machines,  such  care  has  to  be  taken 
in  keeping  the  same  excitation  that  it  is  frequently  undesirable 
to  ground  all  the  neutrals,  but  only  the  neutral  of  one  machine 
is  grounded  and  the  other  machine  neutrals  are  left  isolated.  In 
this  case,  provisions  must  be  made  to  ground  the  neutral  of 
some  other  machine,  if  the  first  one  is  out  of  service.  The 
best  way  is,  when  grounding  generator  neutrals,  to  ground 
through  a  separate  resistance  for  every  generator  and  to 
choose  this  resistance  so  high  as  to  limit  the  neutral  current, 
but  still  low  enough  so  that  in  case  of  a  ground  on  one  phase, 
enough  current  flows  over  the  neutral  to  open  the  circuit 
breaker  of  the  grounded  phase. 

The  use  of  a  resistance  in  the  generator  neutral  is  very 
desirable  also,  since  it  eliminates  the  danger  of  a  high 
frequency  oscillation  between  line  and  ground  through  the 
generator  reactance  in  the  path  of  the  third  harmonic,  by 
damping  the  oscillation  in  the  resistance.  For  this  reason, 
the  resistance  should  be  non-inductive.  To  ground  the  gener- 
ator neutral  through  a  reactance  is  very  dangerous  since  it 
intensifies  the  danger  of  a  resonance  voltage  rise. 

In  grounding  the  generator  neutral,  special  care  is  neces- 
sary to  get  perfect  contact,  since  an  arc  or  loose  contact  would 
generate  a  high  frequency  in  the  circuit  of  the  third  harmonic 
and  so  may  lead  to  a  higher  frequency  oscillation  between  line 
and  ground. 


SEVENTH  LECTURE 


HIGH  FREQUENCY  OSCILLATIONS 
AND  SURGES 

N  an  electric  circuit,  in  addition  to  the  power  consump- 
tion by  the  resistance  of  the  lines,  an  energy  storage 
occurs  as  electrostatic  energy,  or  electrostatic  charge 
due  to  the  voltage  on  the  line  (capacity)  ;  and  as  electromag- 
netic energy,  or  magnetic  field  of  the  current  in  the  line 
(inductance).  In  the  long  distance  transmission  line,  both 
amounts  of  stored  energy  are  very  considerable,  and  of  about 
equal  magnitude;  the  former  varying  with  the  voltage,  the 
latter  with  the  current  in  the  line.  Any  change  of  the  voltage  on 
the  line,  or  the  current  in  the  line,  or  the  relation  between  volt- 
age and  current,  therefore  requires  a  corresponding  change  of 
the  stored  energy;  that  is,  a  readjustment  of  the  stored  energy 

e'C 

in  the  system,  the  electrostatic  energy and  the  electro- 

2 

i'L 

magnetic  energy ,  from  the  previous  to  the  changed  cir- 

2 

cuit  conditions.  This  readjustment  occurs  by  an  oscillation, 
that  is,  a  series  of  waves  of  voltage  and  of  current,  which 
gradually  decreases  in  intensity,  that  is,  dies  out. 

These  oscillating  voltages  and  currents  are  the  result  of 
the  readjustment  of  the  stored  energy  of  the  circuit  to  a  sudden 
change  of  conditions,  and  are  dependant  upon  the  stored  energy 
of  the  circuit,  but  not  upon  the  generator  frequency  or  wave 
shape ;  therefore  they  occur  in  the  same  manner,  and  are  of  the 
same  frequency,  in  a  25  cycle  system  as  in  a  60  cycle  system,  or 
a  high  potential  direct  current  transmission;  and  occur  with 
sine  waves  of  generator  voltage  equally  as  with  distorted 


92  GENERAL  LECTURES 

generator  waves.  While  the  power  of  these  oscillations  ulti- 
mately comes  from  the  generators,  it  is  not  the  generator 
wave  nor  one  of  its  harmonics  which  builds  up,  as  discussed  in 
the  previous  lectures;  but  the  generator  merely  supplies  the 
energy,  which  is  stored  as  electrostatic  charge  of  the  capacity 
and  as  magnetic  field  of  the  inductance,  and  the  readjustment 
of  this  stored  energy  to  the  change  of  circuit  conditions  then 
gives  the  oscillation. 

These  oscillating  voltages  and  currents,  adding  to  the 
generator  voltage  and  current,  thus  increase  the  voltage  and 
the  current  the  more,  the  greater  the  intensity  of  the  oscilla- 
tion, and  so  may  lead  to  destructive  voltages. 

Obviously,  the  intensity  of  the  oscillation,  that  is,  its 
voltage  and  current,  are  the  greater,  the  greater  or  more  abrupt 
the  change  was  in  the  circuit,  which  caused  the  oscillation  by 
requiring  a  readjustment  of  the  energy  storage.  The  greatest 
change  in  a  circuit,  however,  is  the  change  from  short  circuit 
to  open  circuit,  and  the  instantaneous  opening  of  a  short  circuit 
on  a  transmission  line — as  it  occasionally  occurs  by  the  sudden 
rupture  of  a  short  circuiting  arc — therefore  gives  rise  to  the 
most  powerful,  and  thereby  most  destructive  oscillation. 

The  wave  length  of  oscillation  thus  depends  on  the  length 
of  the  circuit  in  which  the  stored  energy  readjusts  itself.  For 
instance,  in  the  short  circuit  oscillation  of  the  system,  the  wave 
extends  over  the  entire  circuit,  including  generators  and  trans- 
formers ;  and  the  entire  circuit  so  represents  one  wave,  or  one- 
half  wave,  that  is,  the  wave  length  is  very  considerable.  If 
the  readjustment  of  stored  energy  takes  place  only  over  a 
section  of  the  circuit,  the  wave  length  is  shorter.  For  instance, 
if  by  a  thunder  cloud  a  static  charge  is  induced  on  the  trans- 
mission line,  and  by  a  lightning  flash  in  the  cloud,  the  cloud 
discharges,  the  electrostatic  charge  induced  by  it  on  the  line 


HIGH  FREQUENCY  OSCILLATIONS  93 

is  set  free  and  dissipates  by  an  oscillation.  In  this  case,  the 
length  of  section  on  which  an  abnormal  charge  existed — one 
mile  for  instance — is  a  half  wave  of  the  oscillation,  and  the 
complete  wave  length  would  thus  be  two  miles.  Or,  if  a 
momentary  discharge  occurs  over  a  lightning  arrester  to 
ground,  the  wave  length  may  be  only  a  few  feet. 

The  velocity  with  which  the  electric  wave  travels  in  an 
overhead  line  is  practically  the  velocity  of  light,  or  about 
188,000  miles  per  second:  it  would  be  exactly  the  velocity  of 
light,  except  that  by  the  resistance  of  the  line  conductor  the 
velocity  is  very  slightly  reduced.  In  an  underground  cable, 
by  the  high  capacity  of  the  cable  insulation,  the  velocity  of 
wave  travel  is  greatly  reduced,  to  about  50  to  70%  of  that  oi 
light. 

From  the  wave  length  and  the  velocity  follows  the  dura- 
tion or  time  of  one  wave,  and  thereby  the  frequency  of  the 
oscillation.  For  instance,  in  the  wave  of  two  miles'  length 
resulting  from  induction  by  a  thunder  cloud,  as  discussed 
above,  the  duration  of  the  wave,  or  the  time  it  takes  to  travel 
the  wave  length  of  two  miles,  at  188,000  miles  per  seconc 

2  I 

velocity,  is =  -       —  second,   and  thus,   during  om 

188,000      94,000 

second,  94,000  waves  would  pass,  that  is,  the  frequency  is 
94,000  cycles.  Or,  if  a  transmission  line  of  80  miles'  length 
short  circuits  at  one  end,  and  then  disconnects  at  the  other 
end  by  the  opening  of  the  circuit  breaker,  in  the  oscillation  pro- 
ducd  thereby  the  circuit  is  one-half  wave.  As  the  length  of  the 
circuit  is  2  x  80  =  160  miles — conductor  and  return  conductor, 
— the  half  wave  is  160  miles;  the  complete  wave  therefore  is 
2  x  1 60  =  320  miles  long,  and  the  duration  of  the  wave  ij 

320  i 

= second ;  the  frequency  587  cycles,  and  if  this 

188,000       587 


94  GENERAL  LECTURES 

short  circuit  oscillation  extends  into,  and  includes  the  generat- 
ing system,  the  frequency  may  be  still  lower. 

Again,  an  oscillation  of  a  very  short  section  of  the  line, 

100  I 

as  for  instance,  100  feet  = = miles  wave  length, 

5280        52.8 


would  have  a  duration  of  the  wave  of 


52.8  x  188,000 
-second,  or  a  frequency  of  9.9  millions  of  cycles  per 


9,900,000 
second. 

Hence  the  frequency  of  such  oscillations,  caused  by  the 
readjustment  of  the  stored  energy  of  the  system,  may  vary 
from  values  as  low  as  machine  frequency,  up  to  many  millions 
of  cycles  per  second.  It  is  the  higher,  the  shorter  the  section 
of  the  circuit  is  in  which  the  readjustment  of  energy  occurs. 
The  higher  the  frequency,  and  therefore  the  shorter  the  section 
of  the  circuit  in  which  energy  readjustment  occurs,  obviously 
the  less  is  the  amount  of  energy  which  is  available  in  the  oscil- 
lation— the  stored  energy  of  this  section — and  the  less  destruc- 
tive therefore  is  the  oscillation.  That  is,  very  high  frequency 
oscillations  are  of  very  low  energy  and  therefore  of  little 
destructiveness;  but  the  energy  and  thus  the  destructiveness 
of  an  oscillation  increases  with  decreasing  frequency,  and  con- 
sequent increasing  extent  of  the  oscillation. 

Such  oscillations  in  a  transmission  line  may  result : 
a.     From  outside  sources,  atmospheric  electric  disturbances, 
as  illustrated  in  the  above  instance. 

b.  They  occur  during  normal  operation  of  the  system: 
any  change  of  load,  or  switching  operation,  as  connecting  or 
disconnecting  circuits,  etc.,  results  in  an  oscillation,  which 
usually  is  so  small  as  to  be  harmless. 


HIGH  FREQUENCY  OSCILLATIONS  95 

c.  It  may  result  from  a  defect  or  fault  in  the  circuit,  as 
an  arcing  ground  or  spark  discharge,  etc. 

One  of  the  most  serious  and  destructive  oscillations  or 
surges  is  that  produced  by  a  spark  discharge  to  ground,  or  an 
arcing  ground,  in  an  overhead  transmission  line  or  an  under- 
ground cable  system. 

Assuming  for  instance  a  44,000  volt  transmission  line  of 
50  miles*  length,  which  is  insulated  from  ground,  that  is,  in 
which  the  neutral  is  not  grounded.  At  44,000  volts  between 
the  line  conductors,  the  voltage  between  each  conductor  and 
the  ground,  normally,  that  is,  with  all  conductors  insulated,  is 

44,000 

=  25,000.     If  now  somewhere  in  the  middle  of  this 

V3 

line  an  insulator  breaks,  and  the  conductor  thus  drops  near  the 
grounded  insulator  pin  or  cross  arm  to  about  2";  with  25,000 
volts  between  conductor  and  ground,  a  spark  would  jump  from 
the  conductor  to  the  ground,  at  the  broken  insulator,  over  the 
2"  §"aP-  This  spark  develops  into  an  arc,  over  which  the 
electrostatic  charge  of  the  conductor  discharges  to  ground  as 
current,  and  the  voltage  of  this  conductor  against  ground  thus 
falls  to  zero,  since  it  is  grounded  by  the  arc;  the  two  other 
line  conductors  then  have  the  full  line  voltage,  of  44,000, 
against  ground ;  and  their  electrostatic  charge  against  ground 
therefore  increases,  from  that  corresponding  to  their  normal 
potential  of  25,000,  to  that  corresponding  to  44,000  volts.  As 
soon  as  the  first  conductor  has  discharged  and  fallen  to  ground 
potential,  the  current  from  this  conductor  to  ground,  over  the 
gap,  ceases,  the  arc  goes  out,  and  the  conductor  so  is  again 
disconnected  from  ground.  It  then  begins  to  charge  again  to  its 
normal  potential  of  25,000  volts  against  ground,  while  the 
other  two  conductors  discharge,  from  44,000  down  to  25,000 


96  GENERAL  LECTURES 

volts.  As  soon,  however,  as  during  the  charge  the  voltage 
of  the  first  conductor  has  risen  to  the  voltage  required  to 
jump  across  a  2"  gap,  this  conductor  again  discharges  to 
ground  by  a  spark,  which  develops  into  an  arc  and  so  on,  the 
phenomena  of  discharge  and  charge  of  the  conductor  repeat- 
ing continuously.  Such  an  oscillation,  which  continues  in- 
definitely, that  is,  until  the  defect  in  the  circuit  is  remedied, 
or  the  circuit  has  broken  down  and  gone  out  of  service,  is 
usually  called  a  surge.  The  duration  of  each  oscillation  of 
such  an  arcing  ground  is  the  time  required :  i .  To  develop  the 
arc,  2.  to  discharge  -the  line,  3.  to  extinguish  the  arc,  4.  to  charge 
the  line.  In  the  above  instance,  the  time  of  charge  or  discharge 
of  the  25  miles  of  line  from  the  arcing  ground  to  the  terminal 

25  I 

station  is : = second.     Assuming  the  velocity  of 

188,000      7520 
the  arc  stream  as  about  2000  feet  per  second,  the  development 

2  I 

or  extinction  of  a  2"  arc  would  require = 

12   X   2000  12,000 

second,  and  the  total  duration  of  one  oscillation  therefore  is : 

i  I  I  I  I 

h 1 H = second,  so  giving  a 

12,000       7520       12,000       7520      2300 

frequency  of  2300  cycles. 

The  two  other  lines  therefore  oscillate  in  voltage  against 
ground,  that  is,  charge  and  discharge  also  at  a  frequency  of 
2300  cycles.  They  receive  their  charge,  however,  over  the 
transformers  at  the  two  ends  of  the  line,  and  their  capacity 
therefore  is  in  series  with  the  self -inductance  of  these  trans- 
formers in  the  circuit  of  the  surge  frequency  of  2300  cycles; 
and  the  voltage  of  the  other  two  lines  thus  may  build  up  by  the 
combination  of  capacity  and  inductance  in  series,  to  excessive 
values ;  that  is,  a  destructive  breakdown  occurs  from  the  other 


HIGH  FREQUENCY  OSCILLATIONS  97 

lines  to  ground — or  in  the  apparatus  connected  to  them  in  the 
terminal  stations  of  the  line,  as  transformers,  current  trans- 
formers, etc. 

A  spark  discharge  or  oscillating  ground  therefore  is  one  of 
the  most  serious,  as  well  as  not  infrequent  disturbances  on  a 
long  distance  transmission  line  or  underground  cable  circuit; 
and  it  is  mainly  as  a  protection  against  this  surge  that  it  is 
recommended  by  many  transmission  engineers  to  ground  the 
neutral  of  the  system  and  so  immediately  convert  a  spark  dis- 
charge on  one  conductor  into  a  short  circuit  of  one  phase  of  the 
system,  and  thereby  automatically  cut  out  the  circuit;  that  is, 
rather  shut  down  this  circuit  than  continue  operation  with  an 
arcing  ground  on  the  system.  Where,  as  in  underground  cable 
systems,  a  number  of  cables  are  used  in  multiple,  the  immediate 
disconnection  of  an  arcing  cable  undoubtedly  is  advisable.  In 
a  single  overhead  transmission  line,  where  a  shutdown  means 
a  discontinuity  of  service,  the  question,  whether  by  grounding 
the  neutral  it  is  preferable  to  shut  down  immediately  in  the 
case  of  an  arcing  ground,  or  continue  service  with  ungrounded 
neutral  and  try  to  find  and  eliminate  the  arcing  ground  on  the 
conductor,  depends  upon  the  length  of  time  which  the  surge 
would  probably  last  before  causing  a  break  down;  it  thereby 
depends  upon  the  character  of  the  circuit;  the  margin  of  in- 
sulation in  transformers  and  insulators ;  and  also  on  the  value 
of  continuity  of  service.  The  question  of  grounding  or  not 
grounding  the  neutral  of  a  transmission  line  therefore  requires 
investigation  in  each  individual  instance. 


EIGHTH  LECTURE 


GENERATION 

For  driving  electric  generators  the  following  methods  are 
available : 

1.  The  hydraulic  turbine  in  a  water  power  station. 

2.  The  steam  engine. 

3.  The  steam  turbine. 

4.  The  gas  engine. 

COMPARISON  OF  PRIME  MOVERS 

i.  The  advantages  of  water  power,  compared  with 
steam  power,  are: 

a.  Very  low  cost  of  operation :  no  fuel,  very  little  attend- 
ance. 

The  disadvantages  are : 

a.  Usually  the  cost  of  development  and  installation  is 
far  higher  than  with  steam  power. 

b.  The  location  of  the  water  power  cannot  be  chosen 
freely,  but  is  fixed  by  nature;  therefore  the  power  cannot  be 
used  where  generated,  but  a  long  distance  transmission  line  is 
required. 

c.  Usually  lower  reliability  of  service,  due  to  the  depend- 
ence on  a  transmission  line,  and  on  meteorological  conditions : 
the  river  may  run  dry  in  summer,  ice  interfere  with  the  opera- 
tion in  winter. 

The  speed  of  the  water  in  the  turbine  depends  upon  the 
head  of  water,  and  is  approximately,  in  feet  per  minute, 
480 Vh,  where  h  is  the  head,  in  feet.  The  perispheral  speed  of 
the  turbine,  and  so  its  revolutions,  depend  upon  the  speed  and 
therefore  upon  the  head  of  the  water.  At  high  heads  of  500  to 


%$&£%  GENERAL  LECTURES 

2000  feet,  as  are  found  in  the  West,  the  electric  generators  are 
thus  high  speed  machines,  of  good  economy  and  moderate  size 
and  cost.  At  low  heads,  however,  such  as  are  usual  in  the  East- 
ern States,  direct  connection  to  a  turbine  leads  to  slow  speed 
generators  of  many  poles  and  large  size  and  cost ;  while  indir- 
ect driving,  by  belt  or  rope,  is  mechanically  undesirable.  Very 
low  head  water  powers  of  less  than  20  to  30  feet  head  there- 
fore are  of  little  value  and  itheir  development  is  economical  only 
where  electric  power  is  valuable. 

Of  the  two  types  of  turbines,  the  reaction  turbine  runs 
approximately  at  the  speed  of  the  water,  and  the  action  or 
impulse  turbine  at  half  the  speed  of  the  water.  At  the  same 
head  and  thus  the  same  speed  of  the  water,  the  reaction  turbine 
gives  higher  speed,  and  is  therefore  used  in  water  powers  of 
low  and  medium  heads,  where  the  speed  of  the  water  is  low; 
while  the  impulse  (turbine,  as  the  Pelton  wheel,  is  always  used 
at  very  high  heads,  at  which  the  reaction  turbine  would  give 
too  high  speeds. 

Where  water  power  is  not  available,  the  power  has  to  be 
generated  by  the  combustion  of  fuel.  In  this  case,  a  greater 
freedom  exists  in  the  choice  of  the  location  of  the  plant;  and 
it  is  located  as  near  to  the  place  of  consumption  as  considera- 
tions of  the  cost  of  property,  the  availability  of  condensing 
water  for  the  engines,  the  facilities  of  transportation,  etc.,  per- 
mit. Transmission  lines  therefore  are  less  frequently  used,  but 
in  steam  stations  of  large  power,  high  potential  distribution  cir- 
cuits of  6600,  11,000  or  13,200  volts,  commonly  underground 
by  cables,  are  used  in  supplying  electric  power  from  (the  main 
generating  station,  to  the  substations  as  centres  of  secondary 
distribution  (New  York,  Chicago,  etc.). 

As  source  of  power  is  available  then : 

The  steam  engine.    The  steam  turbine.    The  gas  engine. 


GENERATION  103 

Comparison  of  the  steam  turbine  with  the  steam  engine: 
Some  of  the  advantages  of  the  steam  turbine  over  the 
steam  engine  are : 

a.  High  efficiency  at  low  loads,  and  a  flatter  efficiency 
curve;  that  is,  the  turbine  efficiency  remains  high  at  partial 
loads,  and  at  overloads,  where  the  steam  engine  efficiency  falls 
off  greatly;  so  that  the  superiority  of  the  steam  turbine  in 
efficiency,  while  marked  at  rated  load,  is  still  far  greater  at 
partial  load,  light  load  and  overload. 

b.  Smaller  size,  weight  and  space  occupied. 

c.  Uniform  rate  of  rotation,  therefore  decreased  liability 
of  hunting  of  synchronous  machines,  and  decreased  necessity 
of  heavy  foundations  to  withstand  reciprocating  strains. 

d.  Greater  reliability  of  operation  and  far  less  attend- 
ance required. 

The  steam  turbine  reaps  a  far  greater  benefit  in  economy 
than  the  steam  engine  from  superheat  of  the  steam,  and  from 
a  high  vacuum  in  the  condenser. 

Some  of  the  disadvantages  of  the  steam  turbine  are : 

a.  It  is  a  new  type  of  machine,  developed  only  within 
the  last  ten  years,  and  operating  engineers  and  attendants 
are  therefore  less  familiar  with  it  than  with  the  reciprocating 
engine ;  and  the  steam  turbine  is  replacing  the  steam  engine  in 
electric  power  plants  so  rapidly,  that  it  is  difficult  to  get  suf- 
ficient men  to  intelligently  install  and  operate  them. 

It  is  therefore  of  greatest  benefit  in  a  steam  turbine  instal- 
lation that  the  user  familiarize  himself  with  the  machine,  so 
as  not  to  depend  upon  the  manufacturer  in  every  minute  detail, 
but  take  care  of  minor  troubles  just  as  he  would  do  with  a  steam 
engine.  As  the  steam  turbine  is  a  very  simple  apparatus  this 
is  not  difficult. 


io4  GENERAL  LECTURES 

The  speed  characteristic  of  the  steam  turbine  is  similar 
to  that  of  the  constant  voltage  direct  current  shunt  motor,  or 
the  polyphase  induction  motor ;  while  that  of  the  reciprocating 
steam  engine  is  similar  to  that  of  the  series  motor.  That  is, 
to  produce  the  same  torque,  the  steam  turbine  requires  approxi- 
mately the  same  amount  of  steam,  irrespective  of  the  speed; 
therefore  its  efficiency  is  highest  at  a  certain  speed,  or  rather 
range  of  speed,  but  falls  off  with  the  speed;  while  the  steam 
consumption  of  the  reciprocating  engine,  at  constant  torque,  is 
approximately  proportional  to  the  speed,  that  is  the  number  of 
times  the  cylinders  are  filled  per  minute.  Or  in  other  words, 
the  torque  per  pound  of  steam  used  per  minute  is  approximately 
constant  and  independent  of  the  speed  in  the  turbine  (just  as 
the  torque  per  volt-ampere  is  approximately  constant  for  all 
speeds  in  the  induction  motor),  while  in  the  reciprocating  en- 
gine the  torque  per  pound  of  steam  used  per  minute  is  approxi- 
mately inversely  proportional  to  the  speed,  or  at  least  greatly 
increases  with  decrease  of  speed  (just  as  in  the  series  motor 
the  torque  per  volt-ampere  input  increases  with  decrease  of 
speed). 

The  steam  turbine  therefore  would  not  be  suitable  for 
directly  driving  a  railway  train  in  rapid  transit  service,  but  is 
suitable  for  driving  the  ship's  propeller. 

Just  as  in  the  induction  motor  a  series  of  economical 
speeds  can  be  produced  by  changing  the  number  of  poles,  so 
in  the  steam  turbine  a  series  of  economical  speeds  can  be  pro- 
duced by  changing  the  number  of  expansions.  For  driving 
electrical  machinery  this,  however,  is  of  no  importance. 

Comparison  of  the  gas  engine  with  the  steam  turbine  and 
the  steam  engine. 

The  leading  and  foremost  advantage  of  the  gas  engine, 
and  the  feature  which  gives  it  the  right  of  existence,  is  its 


GENERATION  105 

high  efficiency.  That  is,  the  same  amount  of  coal,  converted 
to  gas  and  fed  to  a  good  gas  engine,  gives  far  more  power 
than  when  burned  under  the  boilers  of  the  most  efficient  steam 
turbine.  The  cause  is  that  the  gas  engine  works  over  a  far 
greater  temperature  range  than  the  steam  engine  and  even  the 
steam  turbine — although  the  latter,  by  its  ability  to  economic- 
ally utilize  superheat  and  high  condenser  vacuum,  gets  the 
benefit  of  a  larger  temperature  range  over  the  steam  engine. 

If  therefore  the  gas  engine  were  not  so  very  greatly  handi- 
capped in  every  other  respect,  it  would  long  have  superseded 
the  steam  engine  and  the  steam  turbine. 

The  disadvantages  of  the  gas  engine  in  every  respect  but 
efficiency  are  such,  however,  that  in  spite  of  its  existence  of 
over  half  a  century;  it  has  not  made  a  serious  impression  on  the 
industry;  while  the  steam  turbine  in  the  last  ten  years  of  its 
development  has  practically  replaced  the  steam  engine  in  large 
electric  generating  plants. 

The  cause  of  the  disadvantages  of  the  gas  engine  is  the 
high  maximum  temperature  and  the  high  maximum  pressure 
compared  with  the  mean  pressure  in  the  cylinders,  which  is 
necessary  to  get  the  greater  temperature  range  and  thus  the 
efficiency,  therefore  is  inherent  in  this  type  of  apparatus. 

The  output  depends  upon  the  mean  pressure  in  the 
cylinder,  which  is  low;  the  strains  on  the  maximum  pressure, 
which  is  very  high ;  and  the  gas  engine  therefore  must  be  very 
large,  and  its  moving  parts  very  strong  and  heavy,  for  its  out- 
put. The  impulse  due  to  the  rapid  pressure  change  is  very 
jerky — almost  of  the  nature  of  an  explosion — and  the  steadi- 
ness of  the  rate  of  rotation  is  therefore  very  low,  requiring  for 
electric  driving  very  heavy  flywheels  and  numerous  cylinders. 

Compared  with  the  steam  engine,  the  disadvantages  of  the 
gas  engine  so  are : 


106  GENERAL  LECTURES 

a.  Lower  reliability;    higher    cost    of    maintenance    in 
attendance,  repairs,  and  greater  depreciation. 

b.  Larger  size  and  space  occupation  for  the  same  output. 

c.  Less  ease  to  start. 

d.  In  general,  lower  steadiness  of  the  rate  of  rotation. 
The  advantage  of  the  gas  engine  is,  that  it  requires  no 

boiler  plant;  the  compensating  disadvantage,  that  it  requires  a 
gas  generating  plant.  This  latter  disavantage  disappears  where 
gas  is  available  as  fuel — in  the  waste  gases  of  blast  furnaces  of 
steel  plants  and  in  the  natural  gas  districts — and  in  those  cases 
gas  engines  have  found  their  introduction.  They  have  also 
been  installed  for  smaller  powers,  where  low  cost  of  fuel  is  un- 
essential, but  the  operation  of  a  steam  boiler  is  objectionable, 
as  in  isolated  plants  using  city  gas  or  liquid  fuel  (gasolene, 
etc.). 

In  general,  however,  with  the  exception  of  those  special 
cases,  the  gas  engine  does  not  yet  come  into  consideration  in 
the  electric  power  generating  station. 

ELECTRIC  GENERATORS 

In  general,  considerations  of  economy  make  it  desirable 
to  generate  the  electric  power  in  the  form  in  which  it  is  used. 
In  most  cases,  however,  this  is  not  feasible,  but  a  higher 
voltage  or  even  a  different  form  of  power  (alternating  instead 
of  direct)  is  necessary  in  the  generating  station  than  that  re- 
quired by  the  user,  to  enable  transmission  and  distribution; 
and  then  usually  three-phase  alternating  current  is  generated. 

i.  For  isolated  plants,  and  in  general  distribution  of  such 
small  extent  as  to  be  within  range  of  220  volt  distribution, 
220  volt  direct  current  generators  are  used,  operating  a  three- 
wire  system,  either  two  no  volt  machines,  supplying  the 
the  two  sides  of  the  system,  or  220  volt  machines,  deriving  the 


GENERATION 


107 


neutral  by  equalizer  machines,  or  by  connection  to  a  storage 
battery,  or  by  compensator  and  collector  rings  on  the  220  volt 
generator.  That  is,  two  diametrically  opposite  (electrically) 
points  of  the  armature  winding  are  connected  to  collector 
rings,  (so  giving  an  alternating  current  voltage  on  those  col- 
lector rings),  an  alternating  current  compensator  (transformer 
with  a  single  winding)  is  connected  between  the  collector  rings, 
and  the  neutral  brought  out  from  the  center  of  the  compen- 
sator, as  shown  diagrammatically  in  Fig.  24.  This  arrange- 
ment is  now  most  commonly  used. 


Fi£.  24 

For  direct  current  distribution  in  larger  cities,  such 
generating  stations  have  practically  disappeared,  and  have  been 
replaced  by  converter  substations,  receiving  power  from  a 
main  generating  station,  as  three-phase  alternating  current  of 
6600,  11,000  or  13,200  volts,  and  usually  25  cycles. 

2.  For  street  railway,  600  volt  direct  current  generators 
are  still  used  to  a  considerable  extent,  where  the  railway 
system  is  of  moderate  extent.  In  large  railway  systems,  and 
roads  covering  greater  distances,  as  interurban  trolley  lines, 


io8  GENERAL  LECTURES 

direct  generation  of  600  volts  direct  current  is  also  disappear- 
ing before  the  railway  converter  substation,  receiving  power 
as  three-phase  alternating  from  transmission  lines  or  high 
voltage  distribution  cables. 

3.  For  general  distribution  by  alternating  current,  with 
a  2200  volt  primary  system,  direct  generation  is  still  largely 
used,  as  the  use  of  2200  volt  permits  the  system  to  cover  a 
very  large  territory,  and  substations    are   mainly   used    only 
where  the  power  can  be  derived  from  a  long  distance  trans- 
mission line,  or  where  the  2200  volt  distribution  is  only  a  part 
of  a  large  system  of  electric  generation;  as  in  the  suburban 
distribution  of  large  cities,  using  converter  substations  for  the 
interior.    In  this  case,  where  the  transmission  line  or  the  main 
generating  station  is  at  60  cycles,  large  station  transformers 
are  used  for  the  supply  of  the  2200  volt  distribution;  where 
the  power  supply  is  at  25  cycles,  either  frequency  converters, 
or  motor  generators  change  to  60  cycles,  2200  volts. 

4.  For  special  use,  as  for  electrochemical  work,  where  the 
electric  power  is  generated  directly,    different   voltages,    etc., 
may  be  used  to  suit  the  requirements. 

Where  the  power  cannot  be  generated  in  the  form  in 
which  it  is  used,  and  that  is  the  case  in  all  larger  systems,  three- 
phase  alternators  are  almost  universally  used. 

The  single-phase  system  has  the  disadvantage  that  single- 
phase  induction  and  synchronous  motors  and  converters  are 
inferior  to  polyphase  machines,  and  single-phase  alternators 
larger  and  less  efficient,  and  for  lighting,  where  single-phase 
is  preferable,  single-phase  lighting  circuits  can  be  operated 
from  polyphase  alternators. 

Two-phase  also  is  gradually  going  out  of  use,  since  it 
offers  no  advantage  over  the  three-phase,  and  the  three-phase  is 


GENERATION  109 

preferable  for  transmission,  requiring  only  three  conductors, 
while  two-phase  requires  four. 

In  polyphase  alternators  the  flow  of  power  is  constant, 
that  is,  at  any  moment  adding  the  power  of  all  phases  gives  the 
same  value,  while  in  single-phase  alternators  the  power  is  pul- 
sating. 

In  a  polyphase  machine  the  armature  reaction  also  is  con- 
stant, in  a  single-phase  machine,  pulsating;  in  the  latter 
therefore,  in  machines  of  very  large  armature  reaction,  as 
turbo-alternators,  pulsations  of  the  magnet  field,  and  thereby 
loss  in  efficiency,  and  heating  may  result. 

An  alternator  has  armature  reaction  and  self-induction. 

The  armature  reaction  is  the  magnetic  action  of  'the  arma- 
ture current  on  the  field,  that  is,  the  armature  current  demag- 
netizes or  magnetizes  the  field  according  to  its  phase,  and  so 
lowers  or  raises  the  voltage.  Armature  reaction  therefore  is 
expressed  in  ampere  turns. 

Self-induction  is  the  action  of  the  armature  current  in 
producing  magnetism  in  the  armature,  which  magnetism  does 
not  go  through  the  field.  This  magnetism  induces  an  e.  m.  f. 
in  the  armature,  which  opposes  or  assists  ithe  e.  m.  f.  produced 
by  the  field  magnetism,  according  to  the  phase  of  the  armature 
current,  and  so  lowers  or  raises  the  voltage.  Self-induction,  or 
''armature  reactance"  therefore  is  expressed  in  ohms. 

Armature  reaction  and  self-induction  therefore  act  in  the 
same  manner,  lowering  the  voltage  with  lagging  and  raising 
the  voltage  with  leading  current. 

In  calculating  alternators,  either  the  armature  reaction 
and  the  self-induction  can  both  be  considered,  which  makes  the 
calculation  more  complicated;  or  the  armature  reaction  may 
be  neglected  and  the  self-induction  made  so  much  larger  as  to 
allow  for  the  armature  reaction.  This  self-induction  is  then 


no  GENERAL  LECTURES 

called  the  "synchronous  reactance"  and,  combined  with  the 
armature  resistance,  the  "synchronous  impedance"  of  the  ma- 
chine. Or  the  self-induction  may  be  neglected  and  only  the 
armature  reaction  considered,  but  which  is  increased  to  allow 
for  the  self-induction. 

The  last  way  (armature  reaction),  is  used  in  designing 
machines;  the  second  way  (synchronous  reactance)  in  calcula- 
tions with  machines  and  systems. 

In  the  momentary  short  circuit  current  of  alternators, 
however,  the  armature  reaction  and  the  self-induction  must  be 
considered  separately,  since  they  act  differently. 

In  the  moment  of  short  circuiting  an  alternator,  the  self- 
induction  acts  immediately  in  limiting  the  current,  but  not  so 
the  armature  reaction,  because  it  takes  time  before  the  arma- 
ture current  demagnetizes  the  field,  that  is,  the  field  exciting 
winding  acts  as  a  short  circuited  secondary  around  the  field 
poles,  and  retards  the  decrease  of  field  magnetism  resulting 
from  the  demagnetizing  action  of  the  armature  current  by 
inducing  a  current  in  the  field  winding,  which  tends  to  main- 
tain the  field  magetism. 

Therefore  in  the  first  moment  after  the  short  circuit 
the  armature  current  is  limited  by  self-induction  only,  and 
is  therefore  much  larger  than  afterwards,  when  self-induction 
and  armature  reaction  both  act. 

In  machines  of  low  armature  reaction  and  high  self- 
induction,  as  high  frequency  alternators,  the  momentary  short 
circuit  current  is  not  much  larger  than  the  permanent  short 
circuit  current.  In  machines  of  low  self-induction,  that  is,  of 
a  well  distributed  armature  winding,  but  high  armature  reac- 
tion, (that  is,  very  large  output  per  pole,  as  in  steam  turbine 
alternators, )  the  momentary  short  circuit  current  may  be  many 


GENERATION  in 

times  greater  than  the  permanent  value  of  the  short  circuit  cur- 
rent, which  is  reached  after  a  few  seconds. 

In  the  moment  of  short  circuiting  such  an  alternator,  the 
field  current  rises  to  several  times  its  normal  value,  and  becomes 
pulsating,  of  double  frequency.  Gradually  the  armature  cur- 
rent and  the  field  current  die  down  to  their  normal  values.  By 
inserting  non-induotive  resistance  in  the  field  circuit  of  the 
alternator,  the  field  current,  which  is  induced  in  the  moment  of 
short  circuit,  can  be  forced  to  die  out  more  rapidly,  and  the 
armature  short  circuit  current  made  thereby  to  reach  its  final 
value  more  quickly,  that  is,  the  duration  of  the  excessive 
momentary  short  circuit  current  may  be  reduced. 

By  inserting  reactance,  as  choke  coils  or  reactive  coils,  in 
the  armature  circuit  of  the  alternator,  its  momentary  short  cir- 
cuit current  can  be  reduced,  and  this  is  advisable  in  such 
machines  in  which  the  current  otherwise  would  reach  danger- 
ous values.  Since  the  regulation  of  such  alternators  mainly 
depends  upon  the  armature  reaction,  which  is  very  large  com- 
pared with  the  self-induction,  even  a  considerable  external  self- 
induction  inserted  as  reactive  coil  for  limiting  the  momentary 
short  circuit  current  does  not  much  increase  the  combined 
effect  of  armature  reaction  and  self-induction ;  that  is,  does  not 
seriously  affect  the  regulation. 


NINTH  LECTURE 


HUNTING  OF  SYNCHRONOUS  MACHINES 

IROSS  currents  can  flow  between  alternators  due  to  dif- 
ferences in  voltage,  that  is,  differences  in  excitation; 
and  due  <to  differences  in  phase,  that  is,  differences 
in  position  of  their  rotors. 

Cross  currents  due  to  differences  in  excitation  are  watt- 
less currents,  magnetizing  the  under-excited  and  demagnetiz- 
ing the  over-excited  machine. 

Cross  currents  due  to  differences  in  position  are  energy 
currents,  accelerating  the  lagging  and  retarding  the  leading 
machine.  Their  magnetic  action  is  a  distortion  or  a  shift  of  the 
field,  that  is,  they  increase  the  one  and  decrease  the  other  pole 
corner. 

If  two  machines  are  thrown  together  out  of  phase,  or 
brought  out  of  the  phase  by  some  cause  (as  the  beat  of  an  en- 
gine, or  the  change  of  load  of  a  synchronous  motor)  then  the 
two  machines  pull  each  other  in  phase  again,  oscillate  a  few 
times  against  each  other,  which  oscillation  gradually  decreases 
and  dies  out,  and  the  machines  run  steadily. 

If  the  oscillations  do  not  decrease,  but  continue,  the 
machines  are  said  to  be  hunting. 

If  the  oscillation  is  small  it  may  do  no  harm ;  if  it  is 
greater,  it  may  cause  fluctuation  of  voltage,  resulting  in  flick- 
ering of  lights,  etc. ;  if  it  gets  very  large,  it  may  throw  the  ma- 
chines out  of  step. 

Some  causes  of  hunting  are: 

ist.     Magnetic  lag. 

2nd.     Pulsation  of  engine  speed. 

3rd.     Hunting  of  engine  governors. 

4th.     Wrong  speed  characteristic  of  engine. 


n6  GENERAL  LECTURES 

ist.  When  the  machines  move  apart  from  each  other, 
magnetic  attraction  opposes  their  separation.  When  they  pull 
together  again,  magnetic  attraction  pushes  them  together  with 
the  same  force,  so  that  they  would  move  over  the  position  of 
coincidence  in  phase  and  separate  again  in  the  opposite  direc- 
tion just  as  much  as  before. 

Energy  losses  as  friction,  etc.,  retard  the  separation  and 
so  make  them  separate  less  than  before,  every  time  they  do 
so,  that  is,  cause  them  gradually  to  stop  see-sawing. 

If,  however,  there  is  a  lag  in  the  magnetic  attraction,  then 
they  come  together  with  greater  force  than  they  separated,  so 
separate  more  in  the  opposite  direction,  that  is,  the  oscillation 
increases  until  -the  machines  fall  out  of  step,  or  the  further 
increase  of  oscillation  is  stopped  by  the  increasing  energy 
losses. 

This  kind  of  hunting  is  stopped  by  increasing  the  energy 
losses  due  to  the  oscillation,  by  copper  bridges  between  the 
poles,  by  aluminum  collars  around  the  pole  faces,  or  by  a  com- 
plete squirrel  cage  winding  in  the  pole  faces. 

The  frequency  of  this  hunting  depends  on  the  magnetic 
attraction,  that  is,  on  the  field  excitation,  and  on  the  weight  of 
the  rotating  mass.  The  higher  the  field  excitation  the  greater 
is  the  magnetic  force,  that  is,  quicker  the  motion  of  the  ma- 
chine and  therefore  the  higher  the  frequency.  The  greater 
the  weight,  the  slower  it  is  set  in  motion,  that  is,  the  lower  the 
frequency. 

Characteristic  of  this  hunting  therefore  is  that  its  fre- 
quency is  changed  by  changing  the  field  excitation. 

2nd.  If  the  speed  of  the  engine  varies  during  the  rota- 
tion, rising  and  falling  with  the  steam  impulses,  then  the 
alternator  speed  and  the  frequency  also  pulsate  with  a 
speed  equal  to,  or  a  multiple  of  the  engine  speed.  If  now  two 


HUNTING  OF  SYNCHRONOUS  MACHINES   n; 

such  alternators  happen  to  be  thrown  together  so  that  the 
moment  of  maximum  frequency  of  one  coincides  with  the 
moment  of  minimum  frequency  of  the  other,  the  two  machines 
cannot  run  in  perfect  phase  with  each  other,  but  pulsate,  alter- 
natingly  getting  out  of  phase  with  each  other,  coming  together, 
and  getting  out  again  in  the  opposite  direction.  If  the  deviation 
of  the  two  engines  from  uniform  rate  of  rotation  is  very 
little — the  maximum  displacement  of  the  alternator  from  the 
position  of  uniform  rotation  not  more  than  three  electrical 
degrees — the  pulsating  cross  currents,  which  flow  between  the 
alternators,  are  moderate,  and  the  phenomenon  harmless,  as 
long  as  the  oscillation  is  not  cumulative.  An  increase  of  the 
weight  of  the  flywheel  of  the  engine  decreases  -the  speed  pul- 
sation and  thereby  decreases  this  form  of  hunting,  which  is 
(the  most  harmless,  but  increases  the  tendency  to  the  hunting  in 
No.  i  and  No.  3,  and  therefore  is  not  desirable;  but  steadiness 
of  engine  speed  should  be  secured  by  the  design  of  the  engine, 
that  is,  by  balancing  the  different  forces  in  the  engine,  as  die 
steam  impulses  and  the  momentum  of  the  reciprocating  masses, 
so  as  to  give  a  uniform  resultant. 

In  such  a  case,  when  running  from  a  single  alternator, 
driven  by  a  reciprocating  engine  with  moderate  speed  pulsa- 
tion, (therefore  receiving  a  slightly  pulsating  frequency)  a 
synchronous  motor  without  anti-hunting  devices,  but  of  high 
armature  reaction,  and  -therefore  high  stability,  may  run  very 
steadily,  with  no  appreciable  current  pulsation ;  while  the  same 
synchronous  motor,  when  supplied  with  a  squirrel  cage  wind- 
ing in  the  field  pole  faces  as  the  most  powerful  anti-hunting 
device,  may  show  pulsation  in  the  current  supplied,  which  in 
a  high  speed  motor,  of  high  momentum,  may  be  considerable. 
The  cause  is,  that  in  the  former  case  the  synchronous  motor 
does  not  follow  the  pulsation  of  frequency,  but  keeps  constant 


n8  GENERAL  LECTURES 

speed,  while  in  the  latter  case  the  squirrel  cage  winding  forces 
the  motor  to  follow  the  variation  in  frequency  by  accelerat- 
ing and  decelerating,  and  the  pulsation  of  the  current  therefore 
is  not  hunting,  but  energy  current  required  to  make  the  motor 
speed  follow  the  engine  pulsation. 

If  the  frequency  of  oscillation  of  the  machine  (as  deter- 
mined by  its  field  excitation  and  the  weight  of  its  moving 
part)  is  the  same  as  the  frequency  of  engine  impulses,  that  is, 
the  same  as  the  number  of  engine  revolutions  or  a  multiple 
thereof,  then  successive  engine  impulses  will  always  come  at 
the  same  moment  of  the  machine  beat  and  so  continuously 
increase  it:  that  is,  the  machine  oscillation  increases,  or  the 
machine  hunts. 

In  this  case  of  cumulative  hunting  caused  by  the  engine 
impulses,  the  frequency  of  oscillation  agrees  with  the  engine 
oscillation. 

3rd.  If  one  alternator  is  a  little  ahead,  that  is,  takes  a 
little  more  load,  its  engine  governor  regulates  by  reducing  the 
steam,  slowing  down  the  alternator  to  its  normal  position. 
When  slowing  down,  the  flywheel  is  giving  power,  therefore 
the  steam  supply  has  been  reduced  more  than  it  should  be, 
that  is,  the  alternator  drops  behind  and  takes  less  load  until  the 
governor  has  admitted  steam  again. 

In  the  meantime,  while  the  first  alternator  was  behind 
and  took  less  load,  the  second  alternator  had  to  take  the  load, 
that  is,  the  governor  of  the  second  alternator  admitted  more 
steam.  When  the  first  alternator  has  picked  up  again  to  its 
normal  load,  the  second  alternator  gets  too  much  steam  and  its 
governor  must  cut  off,  but  then  cuts  off  too  much,  the  same 
way  as  the  first  alternator  did  before;  so  the  two  governors 
hunt  against  each  other  by  alternatingly  admitting  too  much 
and  too  little  steam. 


HUNTING  OF  SYNCHRONOUS  MACHINES   119 

In  this  case  the  frequency  of  hunting  does  not  depend  on 
the  engine  speed  and  does  not  vary  much  with  the  field  exci- 
tation, but  the  hunting  is  usually  much  less  at  heavy  load  than 
at  light  load.  The  reason  is  that  at  load,  when  the  engines 
take  much  steam,  a  little  change  in  the  steam  supply  does  not 
make  so  much  difference  as  at  light  load,  where  the  engines 
take  very  little  steam,  and  so  a  small  change  of  the  governor 
has  a  great  effect. 

4th.  To  run  in  parallel,  the  speed  of  the  engines  driving 
the  alternators  must  decrease  with  the  load  so  that  the  alter- 
nators divide  the  load. 

If  the  speed  did  not  change  with  the  load,  then  there 
would  be  no  division  of  the  load;  the  one  engine  could  take 
all  the  load,  the  other  nothing. 

If  the  speed  curve  of  the  engine  is  such  that  the  speed  does 
not  fall  off  much  for  light  loads,  -then  the  alternators  will 
not  well  divide  the  load  at  light  loads,  but  hunt  while  running 
in  parallel  at  light  load,  and  steady  down  with  the  load. 

To  distinguish  between  different  kinds  of  hunting: 

ist.  Change  of  frequency  with  change  of  field  excita- 
tion points  to  magnetic  hunting,  especially  if  very  marked. 

2nd.  Equality  of  frequency  with  the  generator  speed 
points  to  engine  hunting. 

3rd.  If  the  synchronous  motor  or  converter  steadies 
down  when  only  one  engine  is  running,  it  points  to  engine 
governor  hunting. 

4th.  Steadiness  of  operation  at  load,  and  unsteadiness  at 
light  load  points  to  governor  hunting,  but  may  also  be  due  to 
engine  and  magnetic  hunting. 

5th.  If  by  disconnecting  one  governor  and  governing 
one  engine  only,  the  hunting  disappears,  then  it  is  due  to 
governor  hunting.  If  it  does  not  disappear,  then  both  gover- 


120  GENERAL  LECTURES 

nors  may  be  diconnected  and  the  engines  run  carefully  without 
governors,  by  throttle.  If  the  hunting  then  disappears,  it  is 
due  to  the  governors;  if  it  does  not  disappear,  it  is  probably 
magnetic  hunting. 

If  by  making  the  field  excitation  of  the  two  alternators 
or  two  converters  that  hunt,  unequal — by  increasing  the  one 
and  decreasing  the  other — the  hunting  disappears  or  decreases, 
it  is  magnetic  hunting. 

In  a  case  of  hunting,  the  following  points  should  be  in- 
vestigated : 

A.     HUNTING  OF  SYNCHRONOUS  MOTORS 
OR  CONVERTERS 

ist.  Count  the  number  of  beats  to  get  the  frequency  of 
hunting.  If  the  beats  periodically  increase  and  decrease,  it 
shows  two  frequencies  of  hunting  superimposed  upon  each 
other.  Then  count  -the  total  number  of  beats  per  minute 
(counting  during  intermissions)  and  count  the  number  of 
intermissions  per  minute. 

The  two  frequencies  are  the  number  of  beats  per  minute, 
plus  and  minus  half  the  number  of  intermissions  or  nodes  per 
minute. 

Instance:  80  beats  per  minute,  10  intermissions  per 
minute.  Frequencies  80  +  5  and  80  —  5  or  85  and  75  beats. 

If  one  of  the  two  frequencies  approximately  coincides 
with  the  engine  speed,  it  can  be  assumed  as  the  engine  speed. 
The  number  of  revolutions  of  the  engine  obviously  should  be 
counted,  also 

2nd.  See  whether  any  machine  in  the  system  runs  at  a 
speed  equal  to  the  observed  frequency  of  hunting.  For 
instance,  a  generator  may  make  75  revolutions  per  minute, 
which  accounts  for  this  frequency. 


HUNTING  OF  SYNCHRONOUS  MACHINES   121 

3rd.  With  several  converters  in  the  same  station  see 
whether  the  station  ammeter  also  hunts. 

If  the  station  ammeter  is  very  steady  and  the  converter 
ammeters  hunt,  the  converters  hunt  against  each  other. 
In  this  case  lowering  the  one  and  raising  the  other  con- 
verter field  and,  if  necessary,  readjusting  the  potential  regula- 
tors, may  stop  the  hunting  by  giving  the  two  machines  differ- 
ent frequencies  of  hunting  which  interfere  with  each  other. 

If  all  three  meters  are  unsteady,  the  converters  may  hunt 
against  each  other  or  hunt  together  against  another  station  or 
against  the  generator.  Then  find  out  whether  the  ammeter 
needles  of  both  converters  go  up  and  down  together  or  one 
goes  up  when  the  other  goes  down. 

4th.  Change  the  field  excitation  and  see  whether  the 
change  of  field  excitation  changes  the  frequency.  See  whether 
a  decrease  of  field  excitation  steadies  it.  Occasionally  hunt- 
ing can  be  stopped  by  lowering  the  field  excitation,  that  is, 
running  with  lagging  current. 

5th.  If  several  converters  of  a  substation  feed  into  the 
same  direct  current  system,  as  the  converters  of  other  sub- 
stations, disconnect  the  direct  current  sides  of  the  converters 
and  see  if  they  still  hunt. 

If  two  or  more  converters  run  in  the  same  station,  run 
only  one  and  see  whether  it  hunts. 

CURE 

ist.  If  the  hunting  is  magnetic  hunting  between  con- 
verters or  synchronous  motors,  it  is  frequently  reduced  by  mak- 
ing the  field  excitation  unequal,  or  putting  a  flywheel  on  one 
converter,  or  belting  some  other  machine  to  it,  or  running  an 
induction  motor  in  the  same  station  or  in  any  other  way  break- 
ing up  the  resonance. 


i22  GENERAL  LECTURES 

2nd.  Several  converters  hunting  against  each  other  in  the 
same  substation  are  frequently  steadied  by  connecting  the 
collector  rings  with  each  other,  that  is,  by  equalizer  connec- 
tions between  converter  and  transformer  or  regulator. 

In  this  case  the  commutator  brushes  have  to  be  carefully 
adjusted  to  avoid  sparking. 

3rd.  The  most  effective  way  is  to  put  copper  bridges  on 
the  converters  or  synchronous  motors,  or  better  still  a  squirrel 
cage  winding  in  the  field  pole  faces. 

Not  so  good  are  short  circuiting  rings  around  the  field 
poles. 

B.     HUNTING  OF  GENERATORS 

ist.    Count  the  frequency  in  the  same  way  as  before. 

2nd.  See  whether  the  frequency  agrees  with  the  genera- 
tor speed  or  with  the  speed  of  some  large  motor  on  the  system. 

3rd.  See  whether  the  frequency  changes  with  the  exci- 
tation. 

4th.  See  whether  the  hunting  changes  with  the  load, 
that  is,  gets  worse  at  light  load. 

5th.  Disconnect  governors  and  see  whether  this  stops 
hunting. 

CURE 

ist.  If  the  hunting  stops  when  disconnecting  the  gover- 
nors, it  is  hunting  of  the  governors  and  can  be  cured  by  putting 
a  stiff  dashpot  on  the  governors. 

2nd.  If  the  hunting  does  not  stop  by  disconnecting  the 
governors,  copper  bridges  on  the  alternators  will  cure  it. 

3rd.  If  the  hunting  has  the  speed  of  the  engine,  it  may 
be  reduced  by  increasing  the  flywheel  or  decreasing  it,  by 
running  an  induction  motor  in  the  station,  or  in  any  other  way 
breaking  up  the  resonance. 


HUNTING  OF  SYNCHRONOUS  MACHINES  123 

In  general,  systems  having  all  kinds  of  loads,  different 
sizes  of  generators,  motors  and  converters,  induction  motors 
and  synchronous  motors  mixed,  etc.,  are  very  little  liable  to 
hunting.  Hunting  is  most  liable  to  occur  when  all  the  genera- 
tors are  of  the  same  kind  and  all  the  synchronous  motors  or 
converters  are  of  the  same  kind. 

Resistance  between  the  machines  increases  the  tendency 
to  hunting  so  that  if  the  resistance  drop  is  more  than  10%  to 
15%,  special  precautions  have  to  be  taken,  such  as  squirrel  cage 
pole  face  windings,  or  synchronous  machines  must  be  alto- 
gether avoided  and  induction  motor  generator  sets  used. 

Reactance  in  general  reduces  the  tendency  to  hunting 
except  when  very  large. 

The  tendency  to  hunting  is  very  severe  at  the  end  of  a 
long  distance  transmission  line  and  induction  machines  as  a 
rule  are  preferable  in  such  a  place. 

Machines  with  high  armature  reaction  are  much  less 
liable  to  hunt  than  machines  with  low  armature  reaction,  that 
is,  close  regulation,  because  with  high  armature  reaction  the 
current  varies  much  less  with  a  change  of  position  of  the 
machine.  Therefore,  60  cycle  converters  are  more  liable  to 
hunt  than  25  cycle  converters,  because  in  60  cycle  converters 
there  is  not  enough  space  on  the  armature  to  get  high  arma- 
ture reaction. 


TENTH  LECTURE 


REGULATION  AND  CONTROL 

A.     DIRECT  CURRENT  SYSTEMS. 

In  direct  current  three-wire  220  volt  distribution  systems 
several  outside  bus  bars  are  used  and,  with  change  of  load,  the 
feeders  are  changed  from  one  bus  bar  to  another. 

The  different  bus  bars  are  connected  to  different  machines, 
to  the  storage  battery  or  to  boosters. 

The  lighting  boosters  are  low  voltage  machines  separ- 
ately excited  from  the  bus  bars.  The  main  generators  are 
shunt  machines  or  rather  are  excited  from  the  bus  bars,  or  ro- 
tary converters,  and  are  usually  of  250  volts,  that  is,  the  neutral 
brought  out  by  collector  rings  and  compensator. 

In  railway  circuits,  in  addition  to  trolley  wire  and  rail 
return,  trolley  feeders  and  ground  feeders,  or  plus  and  minus 
feeders  are  sufficient  for  converter  substations,  and  where  the 
distance  gets  too  great  for  feeders,  another  substation  is 
installed. 

When  using  direct  current  generators,  series  boosters  are 
used  to  feed  very  long  feeders  which  otherwise  would  have 
an  excessive  drop  of  voltage.  In  this  way  feeder  drops  of  200 
to  300  volts  are  taken  care  of  by  the  railway  booster.  Such  a 
large  voltage  drop  is  uneconomical  and  railway  boosters  are 
therefore  used  only  for  small  sections  for  which  it  does  not 
pay  to  install  a  separate  station,  especially  where  the  load  is 
very  temporary,  as  for  instance,  heavy  Sunday  load,  etc. 

Railway  boosters  are  series  machines,  that  is,  the  series 
field  and  the  machine  voltage  therefore  are  proportional  to  the 
current.  In  such  railway  boosters  it  is  necessary  to  take  care  in 
the  booster  design  that  it  does  not  build  up  as  series  generator 


128  GENERAL  LECTURES 

feeding  a  current  through  the  local  circuit  between  a  short 
feeder  and  a  long  feeder,  as  shown  in  Fig*.  25. 


A  series  machine  excites  if  the  resistance  of  its  circuit  is 
less  than  a  certain  critical  value.  To  avoid  such  local  circuit, 
either  the  (trolley  circuit  is  cut  between  the  feeders,  or  the 
boosting  kept  below  the  critical  value. 

If  the  distances  are  too  great  for  boosters,  inverted  con- 
verters in  the  generating  station  are  used  to  change  from 
direct  current  to  alternating  current ;  the  alternating  current  is 
sent  by  step-up  and  step-down  transformers  to  the  substation 
and  changed  to  direct  current  by  rotary  converters. 

If  a  considerable  amount  of  power  is  required  at  a  dis- 
tance, it  is  more  convenient  at  the  generating  station  to  use, 
instead  of  inverted  converters,  double  current  generators,  that 
is,  generators  having  commutator  and  collector  rings. 

If  most  of  the  power  is  used  at  a  distance,  alternating 
current  generators  are  used  with  rotary  converters  and  fre- 
quently one  converter  substation  is  located  in  the  generating 
station. 

Invented  converters  and  double  current  generators  are 
now  used  less,  since  usually  the  systems  are  now  so  large  as  to 


REGULATION   AND    CONTROL  129 

require  most  of  the  power  at  a  distance,  and  therefore  alter- 
nating current  generators  are  used. 

Many  big  systems  have  advanced  from  direct  current  gen- 
erators, through  inverted  converters  and  double  current  gen- 
erators, to  the  present  alternators  feeding  converter  substa- 
tions. 

B.    ALTERNATING  CURRENT  SYSTEMS. 

Generator  Regulation. 

ist.     Close  inherent  regulation. 

This  is  secured  by  low  armature  reaction  and  high 
saturation  so  that  the  voltage  does  not  vary  much  with  the 
load. 

Advantages — 

Simple,  requiring  no  additional  apparatus,  etc. 

Instanteous. 

Disadvan  tages — 

Larger  and  more  expensive  generators  and  when  of  very 
close  regulation,  more  difficult  to  run  in  parallel. 

2nd.     Rectifying  Commutator. 

The  main  current  goes  over  a  commutator,  is  rectified, 
and  the  rectified  current  sent  through  a  series  field.  This  ar- 
rangement is  not  used  any  more. 

Advantage — 

Permits  compounding  and  over-compounding  without 
any  elaborate  apparatus. 

Disadvan  tages — 

Only  limited  power  can  be  rectified,  therefore  suitable 
only  for  smaller  machines. 

Compounds  correctly  only  for  constant  power  factoi ;  that 
is,  if  compounded  for  non-inductive  load,  -the  voltage  drops 
on  inductive  load,  since  inductive  load  requires  a  greater  field 
excitation  than  non-inductive  load. 


130  GENERAL  LECTURES 

Brushes  have  to  be  shifted  with  change  of  power  factor, 
that  is,  change  from  motor  load  to  lighting  load,  etc. ;  other- 
wise commutator  sparks  badly. 

These  machines  therefore  were  good  in  the  early  days 
when  all  the  load  was  lighting  load,  but  are  unsuited  at  present 
for  mixed  load. 

3rd.  Form  D  alternator  or  compensated  alternator  with 
compensating  exciter. 

Exciter  is  connected  direct  and  has  the  same  number  of 
poles  as  the  alternator  so  as  to  be  in  synchronism. 

The  main  current  passes  by  collector  rings  through  the 
exciter  armature,  usually  with  interposition  of  a  transformer 
to  keep  the  high  voltage  away  from  the  exciter. 

The  main  current  is  sent  through  the  exciter  in  such  direc- 
tion that  with  non-inductive  load  its  armature  reaction  slightly 
magnetizes  and  so  raised  the  exciter  voltage.  Then  with 
lagging  current  it  magnetizes  much  more,  raises  the  exciter 
voltage  more,  and  with  leading  current  demagnetizes  or  lowers 
the  exciter  voltage. 

By  adjusting  the  machine  it  can  be  made  to  compound 
perfectly  for  non-inductive,  inductive,  and  leading  current 
load  and  also  can  be  made  to  over-compound  at  constant  speed ; 
so  that  with  the  engine  dropping  in  speed  by  the  load,  it  keeps 
constant  voltage,  or  raises  the  voltage  as  desired. 

Advantages — 

Very  quick  and  very  correct  compensation,  and  if  properly 
adjusted,  the  most  perfect  arrangement  and  not  liable  to  get 
out  of  adjustment. 

Disadvantages — 

More  expensive  machine  and  the  average  engineer  not 
skillful  enough  to  adjust  it. 


REGULATION   AND    CONTROL  131 

4th.     Compensating  Commutator  (Alexanderson). 

Commutator  built  multi-segmental  so  that  it  does  not 
spark  with  fixed  position  of  brushes. 

Brushes  set  to  rectify  completely  at  inductive  load;  at 
non-inductive  load  then  rectify  incompletely,  and  so  the  series 
field  gets  less  current. 

At  leading  current  load  the  rectified  current  is  reversed 
and  so  the  series  field  demagnetizes. 

Advantages — 

Takes  care  of  lag  and  lead. 

Disadvantages — 

Special  generator  and  suitable  only  for  small  and  moder- 
ate sizes. 

5th.     POTENTIAL  REGULATOR. 

Tirrill  Regulator 

Rheostat  in  exciter  field  so  large  that  when  in  circuit  the 
excitation  is  the  lowest,  and  that  when  short  circuited  the  exci- 
tation is  the  highest  ever  required. 

A  potential  magnet  in  the  alternator  circuit  operates  a 
contact  maker  which  continuously  cuts  the  resistance  in  and 
out  again,  so  that  the  contact  maker  is  never  at  rest,  but  always 
cuts  in  and  out,  and  the  average  field  excitation  of  the  exciter 
is  between  maximum  and  minimum. 

If  the  voltage  tends  to  drop,  the  contact  remains  a  shorter 
time  on  the  low  than  on  the  high  position,  and  so  raises  exci- 
tation; if  the  voltage  tends  to  rise,  the  contact  maker  remains 
a  shorter  time  on  the  high  than  on  the  low  position,  and  so 
lowers  excitation. 

Advantages — 

Very  simple. 


132  GENERAL,  LECTURES 

Can  be  applied  to  any  alternator  and  requires  no  special 
adjustment. 

Disadvantages — 

Limited  in  power  by  the  sparking  of  the  contact  maker, 
and  so  can  be  used  only  if  the  regulation  of  the  alternator  is 
not  too  bad  or  the  machine  too  large. 

In  very  large  machines  usually  no  regulating  device  is 
used  but  hand  control  of  the  field  rheostat,  since  in  such  large 
machines  the  load  only  varies  slowly  and  never  changes  much, 
as  for  reasons  of  economy  the  machines  are  run  near  full  load ; 
with  the  change  of  load,  machines  are  shut  down  or  started  up. 

Synchronous  Motors  and  Converters. 

In  an  alternating  current  system  or  part  of  the  system 
containing  large  synchronous  motors  or  converters  the  voltage 
can  be  controlled  by  varying  the  motor  or  converter  field  in  the 
same  way  as  with  alternators,  that  is,  by  Tirrill  Regulator  or 
commutator  and  series  field,  etc. 

POTENTIAL  REGULATORS. 

I.     Compensator  regulator. 

With  step-up  or  step-down  transformers  the  voltage  can 
be  regulated  by  having  different  taps  brought  out  of  the  trans- 
former winding  and  so  get  different  voltages  by  means  of  a  dial 
switch.  Where  no  transformers  are  used  a  compensator  with 
different  voltage  taps  gives  the  same  results. 

The  taps  can  be  brought  out  in  the  primary  or  in  the 
secondary,  whichever  is  the  most  convenient:  in  the  second- 
ary, if  the  primary  is  of  very  high  voltage ;  in  the  primary,  if 
the  secondary  is  of  very  low  voltage  and  large  current. 

Advantages — 

Simplest,  cheapest  and  most  efficient. 


REGULATION  AND  CONTROL  133 

Disadvan  tages — 

Step  by  step  variation  and  sparking  at  the  dial  switch. 

2.  INDUCTION  REGULATORS. 

Built  like  induction  motors  with  stationary  primary  in 
shunt  and  movable  secondary  in  series  to  the  line. 

By  moving  the  secondary  the  voltage  varies  from  lower- 
ing to  raising. 

Induction  regulators  are  usually  three-phase  and  of  larger 
sizes  for  rotary  converters  in  lighting  systems. 

When  single-phase,  the  stationary  member  contains  a 
short  circuited  coil  at  right  angles  to  the  primary.  In  the 
neutral  position  this  coil  acts  as  short  circuited  secondary  to 
the  secondary  coil,  and  so  reduces  its  self-induction. 

Advantages — 

Perfectly  uniform  variation  and  considerable  inductance 
which  is  of  advantage  for  rotary  converters. 

Disadvan  tages — 
High  cost. 

3.  MAGNETO  REGULATORS. 

A  stationary  primary  coil  is  in  shunt  and  a  stationary 
secondary  coil  is  in  series  and  at  right  angles  to  the  primary ;  an 
iron  shuttle  moves  inside  of  the  coils  and  so  turns  the  mag- 
netism of  the  primary  coil  into  the  secondary  coil  either  one 
way  or  the  other. 

On  the  dotted  position  the  primary  sends  the  magnetism 
through  the  secondary  in  opposite  direction  as  in  the  drawn 
position,  in  Fig.  26. 


134 


GENERAL  LECTURES 


Fig.  26 

Advantage — 

Uniform  variation. 

Disadvan  tage — 

More  expensive  than  compensator  regulator. 


ELEVENTH  LECTURE 


LIGHTNING  PROTECTION 

HEN  the  first  telegraph  circuits  were  strung  across  the 
country,  lightning  protection  became  necessary,  and 
was  given  to  these  circuits  at  the  station  by  connecting 
spark  gaps  between  the  circuit  conductors  and  the  ground. 

When,  however,  electric  light  and  power  circuits  made 
their  appearance,  this  protection  against  lightning  by  a  simple 
small  spark  gap  to  ground  became  insufficient,  and  this  addi- 
tional problem  arose :  to  open  the  short  circuit  of  the  machine 
current,  which  resulted  from  and  followed  the  lightning  dis- 
charge. 

This  problem  of  opening  the  circuit  after  the  discharge 
was  solved  by  the  magnetic  blow-out,  which  is  still  used  to  a 
large  extent  on  500  volt  railway  circuits;  by  the  horn  gap 
arrester — a  gap  between  two  horn-shaped  terminals, 
between  which  the  arc  rises,  and  so  lengthens  itself  until  it 
blows  out ;  and  later  on,  for  alternating  current,  the  multi-gap 
between  non-arcing  metal  cylinders,  a  number  of  small  spark 
gaps  in  series  with  each  other,  between  line  and  ground,  over 
which  the  lightning  discharges  -to  ground — the  machine  cur- 
rent following  as  arc,  but  stopped  at  the  end  of  the  half  wave 
of  alternating  current;  but  not  starting  at  the  next  half  wave, 
due  to  the  property  of  these  "non-arcing"  metals  (usually 
zinc-copper  alloys),  to  carry  an  arc  in  one  direction,  but  requir- 
ing an  extremely  high  voltage  to  start  a  reverse  arc. 

These  lightning  arresters  operated  satisfactorily  with  the 
smaller  machines  and  circuits  of  limited  power  used  in  the 
earlier  days,  but  when  large  machines  of  close  regulation,  and 
therefore  of  very  large  momentary  overload  capacity  were  in- 


138  GENERAL  LECTURES 

troduced,  and  a  number  of  such  machines  operated  in  multiple, 
these  lightning  arresters  became  insufficient :  the  machine  cur- 
rent following  the  lightning  discharge  frequently  was  so  enor- 
mous that  the  circuit  did  not  open  at  the  end  of  the  half  wave, 
but  the  arrester  held  an  arc  and  burned  up. 

Furthermore,  the  introduction  of  synchronous  motors, 
and  of  parallel  operation  of  generators,  made  it  essential  that 
the  lightning  arrester  should  open  again  instantly  after  dis- 
charge. For,  if  the  short  circuit  current  over  the  arrester 
lasted  for  any  appreciable  time:  a  few  seconds,  synchronous 
motors  and  converters  dropped  out  of  step,  the  generators  broke 
their  synchronism,  and  the  system  in  this  way  would  be  shut 
down.  The  horn  gap  arrester,  in  which  the  arc  rises  between 
horn-shaped  terminals,  and  by  lengthening,  blows  itself  out, 
therefore  became  unsuitable  for  general  service ;  since  without 
series  resistance,  the  short  circuiting  arc  lasted  too  long  for 
synchronous  apparatus  to  remain  in  step,  and  with  series  resist- 
ance reducing  the  current  so  as  not  to  affect  synchronous  ma- 
chines, it  failed  to  protect  under  severe  conditions.  Thus  it  has 
been  relegated  for  use  as  an  emergency  arrester  on  some  over- 
head lines,  to  operate  only  when  a  shutdown  is  unavoidable. 

To  limit  the  machine  current  which  followed  the  light- 
ning discharge,  and  so  enable  the  lightning  arrester  to  open 
the  discharge  circuit,  series  resistance  was  introduced  in  the 
arrester.  Series  resistance,  however,  also  limited  the  discharge 
current,  and  with  very  heavy  discharges,  such  lightning 
arresters  with  series  resistance  failed  to  protect  the  circuits, 
that  is,  failed  to  discharge  the  abnormal  voltage  without 
destructive  pressure  rise.  This  difficulty  was  solved  by  the 
introduction  of  shunted  resistances,  that  is,  resistances  shunt- 
ing a  part  of  the  spark  gaps.  All  the  minor  discharges  then 
pass  over  the  resistances  and  the  unshunted  spark  gaps,  the 


LIGHTNING  PROTECTION  139 

resistance  assisting  in  opening  the  machine  circuit  after  the 
discharge.  Very  heavy  discharges  pass  over  all  the  spark 
gaps,  as  a  path  without  resistance,  but  those  spark  gaps  which 
are  shunted  by  the  resistance,  open  after  the  discharge;  the 
machine  current,  after  the  first  discharge,  therefore  is  deflected 
over  the  resistances,  limited  thereby ;  and  the  circuit  so  finally 
opened  by  the  unshunted  spark  gaps. 

With  the  change  in  the  character,  size  and  power  of 
electric  circuits,  the  problem  of  their  protection  against  light- 
ning thus  also  changed  and  became  far  more  serious  and 
difficult.  Other  forms  of  lightning,  which  did  not  exist  in  the 
small  electric  circuits  of  early  days,  also  made  their  appear- 
ance, and  protection  now  is  required  not  only  against  the 
damage  threatened  by  atmospheric  lightning,  but  also  against 
"lightning"  originating  in  the  circuits :  so  called  "internal  light- 
ning," which  is  frequently  far  more  dangerous  than  the  dis- 
turbances caused  by  thunder  storms. 

Under  lightning  in  its  broadest  sense  we  now  understand 
all  the  phenomena  of  electric  power  when  beyond  control. 

Electric  power,  when  getting  beyond  control  may  mean 
excessive  currents,  or  excessive  voltages.  Excessive  currents 
are  rarely  of  serious  moment :  since  the  damage  done  by  exces- 
sive currents  is  mainly  due  to  heating,  and  even  very  excessive 
currents  require  an  appreciable  time  before  producing  danger- 
ous temperatures.  Usually  circuit  breakers,  automatic  cut- 
outs, etc.,  can  take  care  of  excessive  currents,  and  such  currents 
produce  damage  only  in  those  instances  where  they  occur  at 
the  moment  of  opening  or  closing  a  switch,  by  burning  con- 
tacts, or  where  the  mechanical  forces  exerted  by  them  are 
dangerously  large,  as  with  the  short  circuit  currents  of  the 
modern  huge  turbo-generators. 


140  GENERAL  LECTURES 

Excessive  voltage,  however,  is  practically  instantaneous 
in  its  action,  and  the  problem  of  lightning  protection  therefore 
is  essentially  that  of  protecting  against  excessive  voltages. 

The  performance  of  the  lightning  arrester  on  an  electric 
circuit  is  analogous  to  that  of  the  safety  valve  on  the  steam 
boiler,  that  is,  to  protect  against  dangerous  pressures — whether 
steam  pressure  or  electric  pressure — by  opening  a  discharge 
path  as  soon  as  the  pressure  approaches  the  danger  limit. 
Therefore  absolute  reliability  is  required  in  its  operation,  and 
discharge  with  as  little  shock  as  possible,  but  over  a  path  amply 
large  to  discharge  practically  unlimited  power  without  danger- 
ous pressure  rise. 

However,  the  causes  of  excessive  pressures,  and  the  forms 
which  such  pressures  may  assume,  are  so  much  more  varied 
in  electric  circuits  than  with  steam  pressures,  that  the  design 
of  perfectly  satisfactory  lightning  arresters  has  been  a  far 
more  difficult  problem  than  the  design  of  the  steam  safety 
valve. 

Such  excessive  pressures  may  enter  the  electric  circuit 
from  the  outside  by  atmospheric  disturbances  as  lightning,  or 
may  originate  in  the  circuit. 

Excessive  pressures  in  electric  circuits  may  be  single 
peaks  of  pressure,  or  "strokes"  or  discharges,  or  multiple 
strokes;  that  is,  several  strokes  following  each  other  in  rapid 
succession,  with  intervals  from  a  small  fraction  of  a  second 
to  a  few  seconds,  or  such  excessive  pressures  may  be  prac- 
tically continuous,  the  strokes  following  each  other  in  rapid  suc- 
cession, thousands  per  second,  sometimes  for  hours. 

Atmospheric  disturbances,  as  cloud  lightning,  usually 
give  single  strokes,  but  quite  frequently  multiple  strokes,  as 
has  been  shown  by  the  oscillograms  secured  of  such  lightning 
discharges  from  transmission  lines.  Any  lightning  arrester 


LIGHTNING  PROTECTION 


141 


to  protect  the  system  must  therefore  be  operative  again  im- 
mediately after  the  discharge,  since  very  often  a  second  and  a 
third  discharge  follows  immediately  after  the  discharge  within 
a  second  or  less. 


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i42  -    GENERAL  LECTURES 

Continuous  discharges,  or  recurrent  surges,  (lightning 
lasting  continuously  for  long  periods  of  time  with  thousands 
of  high  voltage  peaks  per  second),  mainly  originate  in  the 
circuits :  by  an  arcing  ground,  spark  discharge  over  broken 
insulators,  faults  in  cables,  etc.  These  phenomena,  which  have 
made  their  appearance  only  with  the  development  of  the 
modern  high  power  high  voltage  electric  systems,  become  of 
increasing  severity  and  danger  with  the  increase  in  size  and 
power  of  electric  systems. 

Single  strokes  and  multiple  strokes,  that  is,  all  the  dis- 
turbances due  to  atmospheric  electricity,  as  cloud  lightning, 
are  safely  taken  care  of  by  the  modern  multi-gap  lightning 
arrester.  In  its  usual  form  for  high  alternating  voltages,  it 
comprises  a  large  number  of  spark  gaps,  connected  between 
line  and  ground,  and  shunted  by  resistances  of  different  sizes, 
as  shown  in  Fig.  27,  in  such  manner  that  a  high  pressure  dis- 
charge of  very  low  quantity,  as  the  gradual  accumulation  of 
static  charge  on  the  system,  discharges  over  a  path  of  very 
high  resistance  R1,  and  so  discharges  inappreciably  and  even 
frequently  invisibly.  A  disturbance  of  somewhat  higher  power 
finds  a  discharge  path  of  moderate  resistance  R2,  and  so  dis- 
charges with  moderate  current,  that  is.  without  shock  on  the 
system ;  while  a  high  power  disturbance  finds  a  discharge  path 
over  a  low  resistance  R3,  and,  if  of  very  great  power,  even 
over  a  path  of  zero  resistance,  Z.  On  lower  voltage,  commonly 
only  two  resistances  are  used,  one  high  and  one  moderately 
low,  as  shown  by  the  diagram  of  a  2000  volt  multi-gap  arrester, 
Fig.  28. 

The  resistance  of  the  discharge  path  of  the  present  multi- 
gap  arrester  therefore  is  approximately  inversely  proportional 
to  the  volume  of  the  discharge.  This  is  an  essential  and  im- 
portant feature.  Occasionally  discharges  of  such  large  volume 


LIGHTNING  PROTECTION 


occur,  as  to  require  a  discharge  path  of  no  resistance,  as  any 
resistance  would  not  allow  a  sufficient  discharge  to  keep  the 
voltage  within  safe  limits.  At  the  same  time  the  discharge 
should  not  occur  over  a  path  without  a  resistance  or  of  very  low 
resistance,  except  when  necessary,  since  the  momentary  short 


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circuit — that  is,  the  short  circuit  for  a  part  of  the  half  wave — 
of  a  resistanceless  discharge  is  a  severe  shock  on  the  system, 
which  must  be  avoided  wherever  permissible. 

This  type  of  lightning  arrester  takes  care  of  single  dis- 
charges and  of  multiple  discharges,  no  matter  how  frequently 


144  GENERAL  LECTURES 

they  occur  or  how  rapidly  they  follow  each  other,  with  the  mini- 
mum possible  shock  on  the  system.  It  cannot  take  care,  how- 
ever, of  continuous  lightning — those  disturbances,  mainly 
originating  in  the  system,  where  the  voltage  remains  exces- 
sive continuously  (or  rather  rises  thousands  of  times  per 
second  to  excessive  values),  and  for  long  times.  With  such 
a  recurring  surge,  the  multi-gap  arrester  would  discharge  con- 
tinuously in  protecting  the  system,  until  it  destroys  itself  by 
the  excessive  power  of  the  continuously  succeeding  discharges. 
Where  such  continuous  lightning  may  occur  frequently, 
as  in  large  high  power  systems,  and  the  system  requires  pro- 
tection against  them,  a  type  of  lightning  arrester  which  can 
discharge  continuously,  at  least  for  a  considerable  time,  with- 
out self-destruction,  is  necessary.  The  only  lightning  arrester 
which  is  capable  of  doing  this,  is  the  electrolytic,  or  aluminum 
arrester.  In  its  usual  form  (cone  or  disc  type)  it  comprises 
a  series  of  cone-shaped  aluminum  cells,  connected  between  line 
and  ground  through  a  spark  gap.  As  soon  as  the  voltage  of 
the  system  rises  above  normal,  by  the  value  for  which  the 
spark  gap  is  set,  a  discharge  takes  place  through  the  aluminum 
cells,  over  a  path  of  practically  no  resistance;  but  the  volume 
of  the  discharge  which  passes,  is  not  that  given  by  the  voltage 
on  the  system,  but  is  merely  that  due  to  the  excess  voltage  over 
the  normal,  since  the  normal  voltage  is  held  back  by  the  counter 
e.  m.  f.  of  the  aluminum  cells.  As  a  result — with  strokes 
following  each  other,  thousands  per  second,  that  is,  with  a 
recurrent  surge — the  aluminum  arrester  discharges  continu- 
ously; but  it  can  stand  the  continuous  discharge  for  half  an 
hour  or  more  without  damage,  since  it  does  not  carry  the 
short  circuit  current  of  the  system,  but  merely  the  short  circuit 
current  of  the  excess  voltage,  and  so  protects  the  circuit 


LIGHTNING  PROTECTION  145 

against  continuous  lightning  for  a  sufficiently  long  time,  until 
the  cause  of  the  high  voltage  can  be  found  and  eliminated. 

Even  the  cone  type  of  aluminum  arrester  discharges  with 
a  slight  shock  on  the  system,  as  the  voltage  must  rise  to  the 
value  of  the  spark  gap,  before  the  discharge  begins,  and  in 
systems,  in  which  even  a  small  voltage  shock  is  objectionable, 
as  mainly  in  large  underground  cable  systems,  and  also  in 
cases  where  it  is  necessary  to  take  care  of  recurrent  surges 
for  an  indefinite  time,  the  no-gap  aluminum  arrester  becomes 
necessary.  In  principle,  this  type  is  the  same  as  the  cone  type, 
but  the  aluminum  cells  are  connected  between  the  conductors 
and  the  ground  without  any  spark  gap,  that  is,  are  continu- 
ously in  circuit,  taking  a  small  current.  For  this  reason,  the 
cells  are  made  larger,  and  of  different  construction,  so  as  to 
radiate  the  heat  of  the  current  which,  while  small,  would  still 
give  a  harmful  temperature  rise  when  allowed  to  accumulate. 
Being  continuously  in  circuit,  a  no-gap  aluminum  arrester 
allows  no  sudden  voltage  rise  whatever,  however  small  it  may 
be,  that  is,  it  acts  just  like  a  flywheel  on  the  engine:  while  it 
allows  gradual  changes  of  voltages,  any  sudden  change  of 
voltage  is  anticipated  and  cut  off,  just  as  any  sudden  change  of 
speed  by  the  flywheel.  The  no-gap  aluminum  cell  so  can 
hardly  be  called  a  lightning  arrester,  but  rather  fulfills  the  duty 
of  a  shock  absorber,  an  electrical  flywheel  on  the  voltage  of 
the  system,  and  as  such  finds  its  proper  place  on  the  bus  bars  of 
the  station  or  substation,  as  "surge  protector." 

The  three  types  of  apparatus :  the  no-gap  aluminum  cell, 
the  aluminum  cone  arrester,  and  the  multi-gap  lightning 
arrester,  then  are  not  different  types  of  apparatus  intended  for 
the  same  purpose,  but  their  operation  and  proper  field  of  use- 
fulness is  different :  the  multi-gap  arrester  protects  the  system 
against  atmospheric  lightning  and  similar  phenomena;  the 


146  GENERAL  LECTURES 

aluminum  cone  arrester  adds  hereto  protection  against  recur- 
rent surges,  where  such  surges  may  occur  and  the  system  re- 
quires protection  against  them,  and  thus  finds  its  field,  but  at  the 
same  time  requiring  somewhat  more  attention  than  the  multi- 
gap  arrester;  and  the  no-gap  aluminum  cell  should  be 
installed  as  electrical  flywheel  at  the  bus  bars  of  the  station, 
and  in  cable  systems,  usually  in  addition  to  other  protection  on 
lines  and  feeders;  it  requires,  however,  occasional  attention, 
and  continuously  consumes  a  small  amount  of  power. 

Of  other  forms  of  lightning  arresters,  the  magnetic  blow- 
out 500  volt  railway  arrester  is  still  in  use  to  a  large  extent, 
but  is  beginning  to  be  superseded  by  the  aluminum  cell.  The 
multi-gap,  being  based  on  the  non-arcing  or  rectifying  prop- 
erty of  the  metal  cylinders  which  exists  only  with  alternating 
current,  is  not  suitable  for  direct  current  circuits.  In  arc 
light  circuits,  that  is,  constant  current  circuits,  horn  gap 
arresters  with  series  resistance  are  generally  used,  especially 
on  direct  current  arc  circuits,  in  which  the  multi-gap  is  not 
permissible.  In  such  circuits  of  limited  current,  and  very  high 
inductance,  the  series  resistance  is  not  objectionable.  Other- 
wise the  horn  gap  arrester  is  still  occasionally  used  outdoors 
as  emergency  arrester  on  transmission  lines,  set  for  a  much 
higher  discharge  voltage  than  the  station  arrester,  and  then 
preferably  without  series  resistance. 


TWELFTH  LECTURE 


ELECTRIC  RAILWAY 

TRAIN  CHARACTERISTICS 

The  performance  of  a  railway  consists  of  acceleration, 
motion  and  retardation,  that  is,  starting,  running  and  stopping. 
The  characteristics  of  the  railway  motor  are: 

1.  Reliability. 

2.  Limited  available  space,  which  permits  less  margin 
in  the  design,  so  that  the  railway  motor  runs  at  a  higher  temp- 
erature, and  has  a  shorter  life,  than  other  electrical  apparatus. 
The  rating  of  a  railway  motor  is  therefore  entirely  determined 
by  its  heating.    That  is,  the  rating  of  a  railway  motor  is  that 
output  which  it  can  carry  without  its  temperature  exceeding  the 
danger  limit.    The  highest  possible  efficiency  is  therefore  aimed 
at,  not  so  much  for  the  purpose  of  saving  a  few  percent,  of 
power,  but  because  the  power  lost  produces  heat  and  so  reduces 
the  motor  output. 

3.  Very  variable  demands  in  speed.    That  is,  the  motor 
must  give  a  wide  range  of  torque  and  speed  at  high  efficiency. 
This  excludes  from  ordinary  railway  work  the  shunt  motor 
and  the  induction  motor. 

The  power  consumed  in  acceleration  usually  is  many 
times  greater  than  when  running  at  constant  speed,  and  where 
acceleration  is  very  frequent,  as  in  rapid  transit  service,  the 
efficiency  of  acceleration  is  therefore  of  foremost  importance, 
while  in  cases  of  infrequent  stops,  as  in  long  distance  and  inter- 
urban  lines,  the  time  of  acceleration  is  so  small  a  part  of  the 
total  running  time,  that  the  power  consumed  during  accelera- 
tion is  a  small  part  of  the  total  power  consumption,  and  high 
efficiency  of  acceleration  is  therefore  of  less  importance. 


150  GENERAL  LECTURES 

Typical  classes  of  railway  service  are : 

1.  Rapid  transit,  as  elevated  and  subway  roads  in  large 
cities. 

Characteristics  are  high  speeds  and  frequent  stops. 

2.  City  surface  lines,  that  is,  the  ordinary  trolley  car 
in  the  streets  of  a  city  or  town. 

Moderate  speeds,  frequent  stops,  and  running  at  vari- 
able speeds,  and  frequently  even  at  very  low  speeds,  are  char- 
acteristic. 

3.  Suburban    and    interurban    lines.     That    is,    lines 
leading    from    cities    into    suburbs    and    to    adjacent    cities, 
through  less  densely  populated  districts. 

Characteristics  are  less  frequent  stops,  varying  speeds, 
and  the  ability  to  run  at  fairly  high  speeds  as  well  as  low 
speeds. 

4.  Long  distance  and  trunk  line  railroading. 
Characteristics  are:  infrequent  stops,  high  speeds,  and  a 

speed  varying  with  the  load,  that  is,  with  the  profile  of  the  road. 

5.  Special  classes  of  service,  as  mountain  roads  and  ele- 
vators. 

Characteristics  are  fairly  constant  and  usually  moderate 
speed;  a  constant  heavy  load,  so  that  the  power  of  accelera- 
tion is  not  so  much  in  excess  of  that  of  free  running;  and 
usually  frequent  stops.  This  is  the  class  of  work  which  can 
well  be  accomplished  by  a  constant  speed  motor,  as  the  three- 
phase  induction  motor. 

The  rate  of  acceleration  and  rate  of  retardation  is  limited 
only  by  the  comfort  of  the  passengers,  which  in  this  country 
permits  as  high  values  as  2  to  21/2  miles  per  hour  per  second, 
that  is,  during  every  second  of  acceleration,  the  speed  increases 
at  the  rate  of  2  to  21/2  miles  per  hour,  so  that  one  second 
after  starting  a  speed  of  2  to  2a/2  miles  per  hour,  5  seconds 


ELECTRIC   RAILWAY 


after  starting  a  speed  of  5  x  2  to  2V2  =  10  to  i2*/2  miles  per 
hour,  etc.,  is  reached. 

Steam  trains  give  accelerations  of  1/2  mile  per  hour  per 
second  and  less  with  heavy  trains,  due  to  the  lesser  maximum 
power  of  the  steam  locomotive. 

SPEED  TIME  CURVES 

In  rapid  transit,  and  all  service  where  stops  are  so  fre- 
quent that  the  power  consumed  during  acceleration  is  a  large 
part  of  the  total  power,  the  speed  time  curves  are  of  foremost 
importance,  that  is,  curves  of  the  car  run,  plotted  with  the  time 
as  abscissae,  and  the  speed  as  ordinate. 


Choose  for  instance,  a  maximum  acceleration  and  maxi- 
mum braking  of  two  miles  per  hour  per  second,  and  assuming 
a  retardation  of  one-quarter  mile  per  hour  per  second  by  fric- 
tion (that  is,  assuming  that  the  car  slows  down  one-quarter 
mile  per  second,  when  running  light  on  a  level  track)  ;  if  then 
the  time  of  one  complete  run  between  two  stations  is  given 
equal  to  A  B  in  Fig.  29,  the  simplest  type  of  run  consists  of 
constant  acceleration,  from  A  to  C,  on  the  line  A  a,  drawn 


152  GENERAL  LECTURES 

under  a  slope  of  two  miles  per  hour  per  second;  at  C  the 
power  is  shut  off  and  the  car  coasts  on  the  slope  C  D,  of  one- 
quarter  mile  per  hour  per  second,  until  at  D,  where  the  coast- 
ing line  cuts  the  braking  line  bB,  (which  also  is  drawn  at  the 
slope  of  two  miles  per  hour  per  second),  the  brakes  are  applied 
and  the  car  comes  to  rest,  at  B.  As  the  distance  traveled  is 
speed  times  time,  the  area  A  C  D  B  so  represents  >the  distance 
traveled,  that  is,  the  distance  between  the  two  stations,  and  all 
speed  time  curves  of  the  same  type  therefore  must  give  the  same 
area.  During  acceleration,  energy  is  put  into  the  car,  and  stored 
by  its  momentum,  which  is  proportional  to  the  weight  of  the  car 
and  the  square  of  the  speed.  It  is  therefore  at  a  maximum  at 
C.  A  part  of  the  energy  represented  by  the  car  speed  is  con- 
sumed during  coasting  in  overcoming  the  friction ;  the  rest  is 
destroyed  by  the  brakes.  Assuming,  as  approximation,  con- 
stant friction,  the  energy  consumed  by  the  car  friction  on  the 
track,  for  runs  of  the  same  distance,  is  constant,  and  the  energy 
destroyed  by  the  brakes  is  represented  by  the  speed  at  the  point 
B,  where  the  brakes  are  applied.  The  lower  therefore  this 
point  B  is,  the  less  power  is  destroyed  by  the  brakes,  and 
the  more  efficient  is  the  run.  More  accurately,  by  pro- 
longing C  D  to  E  so  that  area  D  E  G  =  B  F  G,  the  area 
A  C  E  F  also  is  the  distance  between  the  stations,  and  E  F 
so  would  be  the  speed  at  which  the  car  arrives  at  the  next 
station,  if  no  brakes  were  applied,  and  the  energy  correspond- 
ing thereto  has  to  be  destroyed  by  the  brakes ;  that  is,  represents 
the  energy  lost  during  the  run,  and  should  be  made  as  small 
as  possible,  to  secure  efficiency. 

The  ratio  of  the  energy  used  for  carrying  the  car  across 
the  distance  between  the  stations — that  is,  energy  consumed  by 
track  friction,  (plus  energy  consumed  in  climbing  grades, 
where  such  exist)  to  the  total  energy  input,  that  is,  track  fric- 


ELECTRIC  RAILWAY 

tion  plus  energy  consumed  in  the  brakes,  is  the  operation 
efficiency  of  the  run. 

As  an  illustration,  a  number  of  such  runs,  for  constant 
time  of  the  run,  of  130  seconds,  and  constant  distance  between 
the  stations,  that  is,  constant  area  of  the  speed  time  diagram, 
are  plotted  in  Figs.  29  to  37. 

i.  Constant  acceleration  of  two  miles  per  hour  per 
second,  coasting  at  one-quarter  mile  per  hour  per  secondhand 
braking  at  two  miles  per  hour  per  second.  Here  the  energy 
consumed  by  the  brakes  is  given  by  the  speed  E  F  =  34. 5  miles 
per  hour,  while  the  maximum  speed  reached  is  60  miles  per 
hour. 


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2,.  Acceleration  and  retardation  at  two  miles  per  hour  per 
second.  Constant  speed  running  between.  Fig.  30.  Compared 
with  i,  (which  is  shown  in  30  in  dotted  lines),  the  maximum 
speed  is  slightly  reduced,  e.  g.,  to  51  miles  per  hour,  but  the 
speed  of  application  of  the  brakes,  and  therefore  the  energy  lost 
in  the  brakes,  is  increased.  That  is,  running  at  constant  speed, 
between  acceleration  and  braking,  is  less  efficient  than  coasting 


154 


GENERAL  LECTURES 


with  decreasing  speed.  Besides  this,  at  the  low  power  required 
for  constant  speed  running,  the  motor  efficiency  usually  is 
aready  lower.  It  therefore  is  uneconomical  to  keep  the  power 
on  the  motors  after  acceleration,  and  more  economical  to  con- 
tinue to  accelerate  until  a  sufficient  speed  is  reached  to  coast 
until  the  brakes  have  to  be  applied  for  the  next  station. 
Obviously,  this  is  not  possible  where  the  distance  between  the 
stations  is  so  great,  that  in  coasting  the  speed  would  decrease 
too  much  to  make  the  time,  and  so  applies  only  to  the  case  of 
runs  with  frequent  stops,  as  rapid  -transit. 

3.     Constant    acceleration    of    one    mile    per    hour    per 
second,  braking  at  two  miles,  coasting  one-quarter  mile.    Dia- 


7 


gram  i  is  shown  in  the  same  figure  31,  for  comparison.  As 
seen,  with  the  lower  rate  of  acceleration,  the  maximum  speed 
is  greater,  and  the  lost  speed,  or  speed  E  F,  which  is  destroyed 
by  the  brakes,  is  greater,  that  is,  the  efficiency  of  the  run  is 
lower. 

4.     Constant  acceleration  and  braking  of  one  mile  per 
hour  per  second,  coasting  at  one-quarter  mile.     In  this  case, 


ELECTRIC  RAILWAY 


the  run  between  the  stations  cannot  be  made  in  130  seconds. 
For  comparison,  i  is  shown  dotted  in  Fig.  32.    Here  the  maxi- 


N 


\\ 


mum  speed  and  the  lost  speed  are  still  greater,  that  is,  the 
efficiency  of  the  run  still  lower,  and  at  least  145  seconds 
are  required.  That  is,  the  higher  the  rate  of  acceleration  and 
of  braking,  the  less  is  the  maximum  speed  required,  and  the 
higher  the  operation  efficiency.  With  constant  acceleration 
up  to  the  maximum  speed,  the  operation  therefore  is  the  more 
efficient  the  higher  a  rate  of  acceleration  and  of  braking  is 
used.  While  very  rapid  acceleration  requires  more  power 
developed  by  the  motor  and  put  into  the  car,  the  time  during 
which  the  power  is  developed  is  so  much  shorter,  that  the 
energy  put  into  the  car,  or  power  times  time  of  power  applica- 
tion, is  less  than  with  the  lower  rate  of  acceleration. 

The  highest  operation  efficiency,  in  the  case  of  frequent 
stops,  therefore  is  produced  by  constant  acceleration  at  the 
highest  permissible  rate,  coasting  without  power,  and  then 
braking  at  the  highest  permissible  rate,  as  given  by  i. 


156  GENERAL  LECTURES 

During  acceleration  at  constant  rate,  from  A  to  C,  the 
motor  however  runs  on  the  rheostat.  That  is,  at  all  speeds 
below  the  maximum,  to  produce  the  same  pull  as  at  the  maxi- 
mum speed  C,  the  motor  consumes  the  same  current  and  so  the 
same  power;  while  the  power  which  it  puts  into  the  train  is 
proportional  ito  the  speed,  and  therefore  is  very  low  at  low 
speeds.  Or  in  other  words,  the  motor  during  constant  acceler- 
ation, consumes  power  corresponding  to  maximum  speed, 
while  the  useful  power  corresponds  to  the  average  speed,  which 
during  A  C  is  only  half  the  maximum;  and  so  only  half  the 
available  power  is  put  into  the  car,  the  other  half  being  wasted 
in  the  resistance,  and  the  motor  efficiency  during  constant 
acceleration  therefore  must  be  less  than  50%. 

Constant  acceleration  up  to  maximum  speed,  while  giving 
the  best  operation  efficiency,  so  gives  a  very  poor  motor 
efficiency  and  thereby  low  total  efficiency,  (the  total  efficiency 
being  the  ratio  of  the  useful  energy  to  the  total  energy  put 
into  the  motors,  that  is,  is  operation  efficiency  times  motor 
efficiency). 

This  is  the  arrangement  necessary  for  a  constant  speed 
motor,  as  the  induction  motor;  but  it  does  not  give  the  best 
total  efficiency,  but  a  better  total  efficiency  is  produced  by 
accelerating  partly  on  the  motor  curve,  that  is,  at  a  decreasing 
rate.  This  sacrifices  some  operation  efficiency,  but  increases 
the  motor  efficiency  greatly,  and  so,  if  not  carried  too  far, 
increases  the  total  efficiency. 

The  speed  time  curves  of  the  motor  are  shown  in  Fig.  33, 
and  the  current  consumption  is  also  plotted  in  this  figure. 
Acceleration  is  constant  from  A  to  M,  on  the  rheostat,  and  at 
constant  current  consumption,  from  M,  onwards,  the  accelera- 
tion decreases,  first  slightly,  then  faster,  but  the  current  also 
decreases,  first  rapidly,  and  then  more  slowly;  and  the 


ELECTRIC  RAILWAY 


efficiency,  plotted  in  Fig.  33,  rises  from  O  at  A,  to  go%  at  M, 
and  then  remains  approximately  constant,  while  -the  speed  still 
increases. 


AlLttl 


££ 


6.  This  gives  the  speed  time  curve  of  the  car,  Fig.  34, 
with  acceleration  on  the  motor  curve  and  with  maximum  values 
of  acceleration  and  braking  2,  the  coasting  value  one-quarter; 
that  is,  the  same  as  i,  and  I  is  shown  in  dotted  lines  in  the 
same  figure.  The  acceleration  is  constant,  on  the  rheostat, 
from  A  to  M ;  at  M  the  rheostat  is  cut  out,  and  the  acceleration 
continues  on  the  motor  curve,  at  a  gradually  decreasing  rate, 
until  at  C  the  power  is  shut  off  and  the  car  coasts  until  the 
brakes  are  applied.  The  area  A  M  C  D  B,  representing  the 
distance  between  the  stations,  is  the  same  as  in  i ;  the  opera- 
tion efficiency  is  somewhat  lower,  but  the  total  current  con- 
sumption, as  shown  by  the  curves  of  current,  shown  together 
with  the  speed  time  curves,  is  much  less,  and  the  power  con- 
sumption therefore  is  less ;  that  is,  the  total  efficiency  is  higher. 


i58 


GENERAL  LECTURES 


7.  Fig.  35  gives  another  speed  time  curve  in  which, 
however,  the  motor  is  geared  for  too  low  a  speed ;  so  the  motor 
curve  is  reached  too  early,  and  the  power  has  to  be  kept  on  for 
too  long  a  time,  to  make  the  run  in  time.  As  seen  from  the 
current  curves,  here  the  loss  in  car  efficiency  by  the  decreased 


fiK 


\ 


M 


ELECTRIC   RAILWAY 


acceleration  on  the  motor  curve  is  greater  than  the  saving 
in  motor  efficiency,  and  the  power  consumption  by  the  motor  is 
greater  than  that  without  running  on  the  motor  curve. 

That  is,  the  total  efficiency  of  operation  is  increased  by 
doing  some  of  the  accelerating  on  the  motor  curve,  but  may 


be  impaired  again  by  carrying  this  too  far.  Usually  the 
rheostat  is  all  cut  out  and  the  acceleration  continues  on  the 
motor  curve,  from  about  half  speed  onwards. 

8.  During  the  first  half  of  the  acceleration  on  the  rheo- 
stat, 6,  when  more  than  half  the  voltage  is  consumed  in  the 
rheostat,  half  the  current  can  be  saved  by  connecting  two 
motors  in  series;  that  is,  by  series  parallel  control  on  the 
motors,  as  shown  in  Fig.  36.  If,  however,  the  series  connec- 
tion of  motors  is  maintained  too  long,  as  shown  in  Fig.  37, 
so  that  the  part  of  the  curve  S  P  gets  -too  long,  the  average 
rate  of  acceleration,  and  so  the  operation  efficiency,  is  greatly 
reduced.  That  is,  the  lost  area  becomes  so  large,  that  the 
speed  at  application  of  the  brakes,  and  so  the  power  lost  in 


i6o 


GENERAL  LECTURES 


brakes,  is  greatly  increased.  Series  connection  of  motors,  for 
efficient  acceleration,  therefore  should  not  be  maintained  for 
any  length  of  time  after  the  rheostat  has  been  cut  out. 


£S 


4 


In  series  parallel  control,  as  shown  in  Figs.  36  and  37, 
some  acceleration  occurs  on  Ihe  motor  curve  in  series  connec- 
tion. That  is,  A  S  is  acceleration  on  the  rheostat,  in  series 
connection,  S  P  acceleration  on  the  motor  curve ;  P  M  on  the 
rheostat  in  parallel  connection,  and  M  P  on  the  motor  curve 
in  parallel  connection.  Compared  with  i,  which  is  shown 
dotted  in  9,  the  area  A  S  P  M  H  d  is  lost;  and  so  the  equal 
area  H  C  D  D±,  has  to  be  gained,  giving  a  higher  speed  of 
application  of  the  brakes  D,  but  gaining  power  more  than  the 
increased  power  consumption  in  the  brakes,  by  the  higher 
motor  efficiency. 

CONCLUSION 

In  short  distance  runs  the  efficiency  is  highest  in  running 
on  series  parallel  control  as  much  as  possible  on  the  motor 


ELECTRIC   RAILWAY  161 

curve,  with  as  high  a  rate  of  average  acceleration  and  retarda- 
tion as  possible,  and  coasting  between  acceleration  and  retarda- 
tion ;  that  is,  not  keeping  the  power  on  longer  than  necessary. 

The  longer  the  distance,  the  less  important  is  high  rate 
of  acceleration  and  retardation,  and  for  long  distance  running 
the  rate  of  acceleration  and  retardation  is  of  little  importance. 

Therefore  speed  time  curves  are  specially  important  in 
rapid  transit  service,  and  in  general,  in  running  with  frequent 
stops. 

The  heating  of  the  motor  at  high  acceleration,  that  is, 
with  large  current,  is  less  than  with  low  acceleration,  that  is, 
smaller  current,  because  the  current  is  on  a  much  shorter  time. 

Feeding  back  in  the  line  by  using  the  motors  as  genera- 
tors is  rarely  used ;  because  with  an  efficient  speed  time  curve, 
using  coasting,  the  speed  when  putting  on  the  brakes  is  already 
so  low  that  usually  not  enough  power  can  be  saved  to  compen- 
sate for  the  complication  and  the  increased  heating  of  the 
motors,  when  carrying  current  also  in  stopping.  The  motors  are 
occasionally  used  as  brakes,  operating  as  generators  on  the 
rheostat.  This,  however,  puts  an  additional  heating  on  the 
motors ;  and  is  therefore  not  much  used  in  this  country,  where 
the  highest  speed  which  the  motor  equipment  can  give  is 
desired. 

With  induction  motors,  feeding  back  in  the  line  is 
simplest,  because  induction  motors  become  generators  above 
synchronism,  and  so  feed  back  when  running  down  a  long 
hill.  Therefore  on  mountain  railways,  induction  motors 
have  the  advantage. 

In  an  induction  motor  there  is  no  running  on  the  motor 
curve,  and  so  the  efficiency  of  acceleration  is  lower. 

Objection  to  the  series  motor  is  the  unlimited  speed ;  that 
is,  when  running  light,  it  runs  away.  In  railroading  this  is  no 


1 62  GENERAL  LECTURES 

objection,  because  the  motor  is  never  running  light  and  some- 
body is  always  in  control. 

In  elevator  work  the  series  motor  is  objectionable,  due 
to  the  unlimited  speed ;  therefore  a  limited  speed  motor  is  neces- 
sary. In  elevators  frequent  stops,  and  so  efficient  acceleration 
are  necessary;  therefore  a  compound  motor  is  best,  that  is,  a 
motor  having  a  shunt  field  to  limit  the  speed  and  a  series  field 
(which  is  ctit  out  after  starting)  to  give  efficient  acceleration. 


THIRTEENTH  LECTURE 


ELECTRIC  RAILWAY:      MOTOR 
CHARACTERISTICS 

HE  economy  of  operation  of  a  railway  system,  station, 
lines,  etc.,  decreases,  and  the  amount  of  apparatus,  line 
copper,  etc.,  which  is  required,  increases  with  increas- 
ing fluctuations  of  load ;  the  best  economy  of  an  electric  system 
therefore  requires  as  small  a  power  fluctuation  as  possible. 

The  pull  required  of  the  railway  motor  during  accelera- 
tion, on  heavy  grades,  etc.,  is,  however,  many  times  greater 
than  in  free  running.  In  a  constant  speed  motor,  as  a  direct 
current  shunt  motor  or  an  alternating  current  induction  motor, 
the  power  consumption  is  approximately  proportional  to  the 
torque  of  the  motor  and  thus  to  the  draw  bar  pull  that  is  given 
by  it.  With  such  motors,  the  fluctuation  of  power  consump- 
tion would  thus  be  as  great  as  the  fluctuation  of  pull  required. 
In  a  varying  speed  motor,  as  the  series  motor,  the  pull  increases 
with  decreasing  speed;  and  the  power  consumption,  which  is 
approximately  proportional  to  pull  times  speed,  varies  less 
than  the  pull  of  the  motor.  The  fluctuation  of  load  produced 
in  the  circuit  by  a  series  motor  therefore  is  far  less  than  that 
produced  by  a  shunt  or  induction  motor — the  former  economiz- 
ing power  at  high  pull  by  a  decrease  of  speed ;  the  series  motor 
thus  gives  a  more  economical  utilization  of  apparatus  and  lines 
than  the  shunt  or  induction  motor,  and  is  therefore  almost  ex- 
clusively used. 

The  torque,  and  so  the  pull  produced  by  a  motor,  is 
approximately  proportional  to  -the  field  magnetism  and  the 
armature  current;  that  is,  neglecting  the  losses  in  the  motor, 
or  assuming  100%  efficiency,  the  torque  is  proportional  to  the 
product  of  magnetic  field  strength  and  armature  current. 


1 66 


GENERAL  LECTURES 


In  a  shunt  motor,  at  constant  supply  voltage  e,  the  field 
exciting  current,  and  thus  the  field  strength,  is  constant;  and  the 
torque,  when  neglecting  losses,  is  thus  proportional  to  the 
armature  current,  as  shown  by  the  curve  T0  in  Fig.  38.  From 


this  torque  is  subtracted  the  torque  consumed  by  friction  losses, 
coreloss,  etc.  (which,  at  approximately  constant  speed  and  field 
strength,  is  approximately  constant  and  is  shown  by  the  curve 
TI)  thus  giving  as  net  torque  of  the  motor,  the  curve  T.  Neg- 
lecting losses,  the  speed  of  the  motor  would  be  constant,  as 
given  by  line  S9 ;  since  at  constant  field  strength,  to  consume  the 
same  supply  voltage  e0,  the  armature  has  to  revolve  at  the  same 
speed.  As,  however,  with  increasing  load  and  therefore  in- 
creasing current,  the  voltage  available  for  the  rotation  of  the 
armature  decreases  by  the  ir  drop  in  the  armature,  as  shown 
by  the  curve  e  at  constant  field  strength,  the  speed  decreases 
in  the  same  proportion,  as  shown  by  the  curve  Si.  The  field 
strength,  however,  does  not  remain  perfectly  constant,  but  with 


MOTOR  CHARACTERISTICS 


167 


increasing  load  the  field  magnetism  slightly  changes:  it  de- 
creases by  field  distortion  and  demagnetization,  and  the  speed 
therefore  increases  in  the  same  proportion,  to  the  curve  S.  The 
current  used  as  abscissae  in  Fig.  38  is  the  armature  current. 
The  total  current  consumed  by  the  motor  is,  however,  slightly 
greater,  namely,  by  the  exciting  current  i0 ;  and,  plotted  for  the 
total  current  of  the  motor  as  abscissae,  all  the  curves  in  Fig. 
38  are  therefore  shifted  to  the  right,  by  the  amount  of  i0,  as 
shown  in  Fig.  39. 


me. 


If  in  the  shunt  motor,  the  supply  voltage  changes,  the 
field  strength,  which  depends  upon  ithe  supply  voltage,  also 
changes ;  it  decreases  with  a  decrease  of  the  supply  voltage,  and 
the  current  required  to  produce  the  same  torque  therefore  in- 
creases in  the  same  proportion.  If  the  magnetic  field  is  below 
saturation,  the  field  strength  decreases  in  proportion  to  the  de- 
crease of  supply  voltage,  and  the  current  thus  increases  in  pro- 
portion to  the  decrease  of  supply  voltage,  while  the  speed  re- 


1 68  GENERAL  LECTURES 

mains  the  same,  the  armature  produces  the  lower  voltage  by 
revolving  in  the  lower  field  at  the  same  speed.  If  the  magnetic 
field  is  highly  over-saturated  and  does  not  therefore  appreciably 
change  with  a  moderate  change  of  supply  voltage  and  so  of 
field  current,  the  armature  current  required  to  produce  the 
same  torque  also  does  not  appreciably  change  with  a  moder- 
ate drop  of  supply  voltage,  but  the  speed  decreases,  since  the 
armature  must  now  consume  less  voltage  in  the  same  field 
strength. 

Depending  on  the  magnetic  saturation  of  the  field :  with 
a  decrease  of  the  supply  voltage  the  current  consumed  by  the 
shunt  motor  to  produce  the  same  torque,  therefore  increases 
the  more,  the  lower  the  saturation,  and  the  speed  decreases  the 
more,  the  higher  the  saturation. 

In  general,  a  drop  of  voltage  in  the  resistance  of  lines 
and  feeders  does  not  much  affect  the  speed  of  the  shunt 
motor,  but  increases  the  current  consumption,  thus  still  further 
increasing  the  drop  of  voltage ;  so  that  in  a  shunt  motor  system, 
lines  and  feeders  must  be  designed  for  a  lower  drop  in  voltage 
than  is  permissible  for  a  series  motor. 

The  three-phase  induction  motor  in  its  characteristics  cor- 
responds to  a  shunt  motor  with  under-saturated  field,  except 
that  the  effect  of  a  drop  of  voltage  is  still  more  severe ;  as  not 
only  the  amount,  but  usually  the  lag  of  current  also  increases, 
thus  causing  more  drop  in  voltage ;  and  the  maximum  torque  of 
the  motor  is  limited,  and  decreases  with  the  square  of  the 
voltage.  Hence,  while  in  a  series  motor  system  the  lines  and 
feeders  are  designed  for  the  average  load  or  average  voltage 
drop  (and  practically  no  limit  exists  to  the  permissible  maxi- 
mum voltage  drop),  with  an  induction  motor,  the  maximum 
permissible  voltage  drop  is  limited  by  the  danger  of  stalling 
the  motors. 


MOTOR  CHARACTERISTICS 


169 


In  the  series  motor,  the  armature  current  passes  through 
the  field,  and  with  increasing  load  and  thus  increasing  current, 
the  field  strength  also  increases ;  the  torque  of  the  motor  there- 
fore increases  in  a  greater  proportion  than  the  current.  Neg- 


1 70  GENERAL  LECTURES 

lecting  losses  and  saturation,  the  field  strength  is  proportional 
to  the  current;  the  torque  being  proportional  to  the  current 
times  field  strength,  therefore  is  proportional  to  the  square  of 
the  current,  as  shown  by  the  curve  T0  in  Fig.  40.  The  supply 
voltage,  however,  has  no  direct  effect  on  the  torque;  but  with 
the  same  current  consumption,  the  motor  gives  the  same 
torque,  regardless  of  the  supply  voltage.  The  speed,  at  con- 
stant supply  voltage,  changes  with  the  field  strength  and  thus 
with  the  current :  the  higher  the  field  strength,  the  lower  is  the 
speed  at  which  the  armature  consumes  the  voltage.  Since  the 
field  strength — neglecting  losses  and  saturation — is  propor- 
tional to  the  current,  the  speed  of  the  series  motor  would  be 
inversely  proportional  to  the  current,  as  shown  by  the  curve 
S0  in  Fig.  40. 

As  the  voltage  available  for  the  armature  rotation 
decreases  with  increasing  current,  from  e0  to  e,  by  the  ir  drop 
in  the  field  and  armature,  the  speed  decreases  in  the  same  pro- 
portion, from  the  curve  S0  to  the  curve  Si. 

In  reality,  however,  the  field  strength,  as  shown  by  the 
curve  MO,  is  proportional  to  the  current  only  at  low  currents ; 
but  for  higher  currents  the  field  strength  drops  below,  by  mag- 
netic saturation,  as  shown  by  the  curve  M ;  and  ultimately,  at 
very  high  currents,  it  becomes  nearly  constant.  In  the  same 
ratio  as  the  field  strength  drops  below  proportionality  with  the 
current,  the  speed  increases  and  the  torque  decreases.  The 
actual  speed  curve  is  therefore  derived  from  the  curve  Si  by  in- 
creasing the  values  of  the  curve  Si  in  the  proportion,  M0  to  M, 
and  is  given  by  the  curve  S;  and  in  the  same  proportion  the 
torque  is  decreased  to  the  curve  TI.  From  this  torque  curve 
the  lost  torque  is  now  subtracted ;  that  is,  the  torque  represent- 
ing the  power  consumed  in  friction  and  gear  losses,  hysteresis 
and  eddy  currents,  etc.  Some  of  the  losses  of  power  are 


MOTOR  CHARACTERISTICS  171 

approximately  constant ;  others  are  approximately  proportional 
to  the  square  of  the  current;  and  the  lost  -torque,  being  equal 
to  the  power  loss  divided  by  the  speed,  can  therefore  be  assumed 
as  approximately  constant :  somewhat  higher  at  low  and  high 
speeds,  as  shown  by  curve  F.  The  net  torque  then  is  given  by 
the  curve  T.  As  seen,  it  is  approximately  a  straight  line,  pass- 
ing through  a  point  I0,  which  is  the  "running  light  current," 
and  its  corresponding  speed,  the  "free  running  speed"  of  the 
motor.  At  this  current  i0,  the  speed  is  highest ;  with  increase 
of  current  it  drops  first  very  rapidly,  and  then  more  slowly; 
and  the  higher  the  saturation  of  the  motor  field  is,  the  slower 
becomes  the  drop  of  speed  at  high  currents. 

The  single-phase  alternating  current  motors  are  either 
directly  or  inductively  series  motors,  and  so  give  the  same 
general  characteristics  as  the  direct  current  series  motor.  In 
the  alternating  current  motors,  however,  in  addition  to  the  ir 
drop  an  ix  drop  exists ;  -that  is,  in  addition  to  the  voltage  con- 
sumed by  the  resistance,  still  further  voltage  is  consumed  by 
self-induction;  and  the  voltage  e  available  for  the  armature 
rotation  thus  drops  still  further,  as  seen  in  Fig.  41.  Since  the 
self-induction  consumes  voltage  in  quadrature  with  the  cur- 
rent, the  inductive  drop  is  not  proportional  to  the  current,  but  is 
small  at  low  currents,  and  greater  at  high  currents ;  e  therefore 
is  not  a  straight  line,  but  curves  downwards  at  higher  currents. 
The  speed,  Si,  is  dropped  still  further  by  the  inductive  drop 
of  voltage,  to  the  curve  Si,  and  then  raised  to  the  curve  S  by 
saturation.  The  effect  of  saturation  in  the  alternating  current 
motor  usually  is  far  less,  since  the  magnetic  field  is  alternating, 
and  good  power  factor  requires  a  low  field  excitation,  and 
therefore  high  saturation  cannot  well  be  reached.  The  torque 
curves  are  the  same  as  in  the  direct  current  motor,  except  that 
the  effect  of  saturation  is  less  marked. 


T72 


GENERAL  LECTURES 


In  efficiency,  the  shunt  or  induction  motor,  and  the  series 
motor  are  about  equal ;  and  both  give  high  values  of  efficiency 
over  a  wide  range  of  current.  A  wide  range  of  current,  how- 
ever, represents  a  wide  range  of  speed  in  the  series  motor,  and 


MOTOR  CHARACTERISTICS  173 

nearly  constant  speed  in  the  shunt  motor;  therefore  while  the 
series  motor  can  operate  at  high  efficiency  over  a  wide  range 
of  speed,  the  shunt  motor  shows  high  efficiency  only  at  its 
proper  speed. 

In  regard  to  the  effect  of  a  change  of  supply  voltage,  as  is 
caused,  for  instance,  by  a  drop  of  voltage  in  feeders  and  mains, 
/the  series  motor  reacts  on  a  change  of  voltage  by  a  correspond- 
ing change  of  speed,  but  without  change  of  current ;  while  the 
shunt  motor  and  induction  motor  reacts  on  a  change  of  supply 
voltage  by  a  change  of  current,  with  little  or  no  change  of 
speed.  As  the  limitation  of  a  system  usually  is  the  current, 
at  excessive  overloads  on  the  system,  resulting  in  heavy 
voltage  drop,  the  series  motors  run  slower,  but  continue  to 
move ;  while  the  induction  motor  is  liable  to  be  stalled. 


FOURTEENTH  LECTURE 


ALTERNATING  CURRENT  RAILWAY  MOTOR. 

N  a  direct  current  motor,  whether  a  shunt  or  a  series 
motor,  the  motor  still  revolves  in  the  same  direction, 
if  the  impressed  e.  m.  f.  be  reversed,  as  field  and  arma- 
ture both  reverse.  Since  a  reversal  of  voltage  does  not  change 
the  operation  of  the  motor,  such  a  direct  current  motor  there- 
fore can  operate  also  on  alternating  current.  With  an  alter- 
nating voltage  supply,  the  field  magnetism  of  the  motor  also 
alternates ;  the  motor  field  must  therefore  be  laminated,  to  avoid 
excessive  energy  losses  and  heating  by  eddy  currents  (cur- 
rents produced  in  the  field  iron  by  the  alternation  of  the  mag- 
netism) just  as  in  the  direct  current  motor  the  armature  must 
be  laminated. 

In  the  shunt  motor — in  which  the  supply  current  divides 
between  field  and  armature — when  built  for  alternating  voltage, 
arrangements  must  be  made  to  have  the  current  in  the  field  (or 
rather  the  field  magnetism)  and  the  current  in  the  armature, 
reverse  simultaneously.  In  the  series  motor,  in  which  the  same 
current  traverses  field  and  armature,  the  field  magnetism  and 
the  armature  current  are  necessarily  in  phase  with  each 
other,  or  nearly  so.  Only  the  series  or  varying  speed  type  of 
alternating  current  commutator  motor  has  so  far  become  of 
industrial  importance. 

In  the  alternating  current  motor  in  addition  to  the  voltage 
consumed  by  the  resistance  of  the  motor  circuit  and  that  con- 
sumed by  the  armature  rotation,  voltage  is  also  consumed  by 
self-induction;  that  is,  by  the  alternation  of  the  magnetism. 
The  voltage  consumed  by  the  resistance  represents  loss  of 
power,  and  heating,  and  is  made  as  small  as  possible  in  any 


178  GENERAL  LECTURES 

motor.  The  voltage  consumed  by  the  rotation  of  the  arma- 
ture, or  "e.  m.  f.  of  rotation,"  is  that  doing  (the  useful  work  of 
the  motor,  and  so  is  an  energy  voltage,  or  voltage  in  phase 
with  the  current;  just  as  the  voltage  consumed  by  the  resist- 
ance is  in  phase  with  the  current.  The  voltage  consumed  by 
self-induction,  due  to  the  alternation  of  the  magnetism,  or 
"e.  m.  f.  of  alternation",  is  in  quadrature  with  the  current,  or 
wattless;  that  is,  it  consumes  no  power,  but  causes  the  current 
to  lag,  and  so  lowers  the  power  factor  of  the  motor;  that  is, 
causes  the  motor  to  take  more  volt-amperes  than  corresponds 
to  its  output,  and  so  is  objectionable. 

The  useful  voltage,  or  e.  m.  f.  of  rotation  of  the  motor, 
is  proportional  to  the  speed ;  or  rather  the  "frequency  of  rota- 
tion", NO,  is  proportional  to  the  field  strength  F,  and  to  the 
number  of  armature  turns  m.  The  wattless  voltage,  or  self- 
induction  of  the  field,  is  proportional  to  the  frequency  N,  to 
the  field  strength  F,  and  the  number  of  field  turns  n.  The 
ratio  of  the  useful  voltage  to  the  wattless  voltage  therefore  is 
mN0  -T-  nN,  and  to  make  the  useful  voltage  high  and  the 
wattless  voltage  low,  therefore  requires  as  high  a  frequency  of 
rotation  N0  and  as  low  a  frequency  of  supply  N,  as  possible. 
Thus  the  commutator  motors  of  more  than  25  cycles  give 
poor  power  factors;  and  for  a  given  number  of  revolutions 
NO,  which  is  number  of  revolutions  per  second  times  number 
of  pairs  of  poles,  therefore  is  the  higher,  the  more  poles  the 
motor  has.  Hence  a  greater  number  of  poles  are  generally 
used  in  an  alternating  current  -than  in  a  direct  current  motor. 

Good  direct  current  motor  design  requires  a  strong  field 
and  weak  armature,  to  get  little  field  distortion  and  therefore 
good  commutation ;  that  is  high  n  and  low  m.  But  such  pro- 
portions, even  at  low  supply  frequency  N  and  high  frequency 
of  rotation  N0,  would  give  a  hopelessly  bad  power  factor,  and 


ALTERNATING  CURRENT  MOTOR     179 

thus  a  commercially  impractical  motor.  In  the  alternating  cur- 
rent commutator  motor,  it  is  therefore  essential  to  use  as  strong 
an  armature  and  as  weak  a  field  (that  is,  as  large  a  number  of 
armature  turns  m  and  as  low  a  number  of  field  turns  n)  as  pos- 
sible. Very  soon,  however,  a  limit  is  reached  in  this  direction, 
even  if  the  greater  field  distortion  and  the  resultant  bad  com- 
mutation were  not  to  be  considered :  the  armature  also  has  a 
self-induction ; ;  that  is,  .the  alternating  magnetism  produced  by 
the  current  in  the  armature  turns  consumes  a  wattless  e.  m.  f. 
This  magnetism  is  small  in  a  direct  current  motor,  but  with 
many  armature  turns  and  few  field  turns  it  becomes  quite  con- 
siderable ;  and  so,  while  a  further  decrease  of  the  field  turns  and 
increase  of  the  armature  turns  reduces  the  self-induction  of 
the  field — which  varies  with  the  square  of  the  field  turns — it 
increases  the  self-induction  of  the  armature — which  varies 
with  the  square  of  the  armature  turns.  There  is  thus  a  best 
proportion  between  armature  turns  and  field  turns,  which  gives 
ithe  lowest  total  self-induction.  This  is  about  in  this  propor- 
tion :  armature  turns  m  to  field  turns  n  =  2  -=-  i ;  and  at  this 
proportion  the  power  factor  of  the  motor,  especially  at  low  and 
moderate  speeds,  is  still  very  poor. 

In  alternating  current  commutator  motors  it  is  therefore 
essential  to  apply  means  to  neutralize  the  armature  self-induc- 
tion and  armature  reaction,  so  as  to  be  able  to  increase  the 
proportion  of  armature  turns  to  field  turns  sufficiently  to  get 
good  power  factors.  This  is  done  by  surrounding  the  arma- 
ture with  a  stationary  "compensating  winding"  closely  adja- 
cent to  the  armature  conductors,  located  in  the  field  pole  faces, 
and  traversed  by  a  current  opposite  in  direction  to  the  current 
in  the  armature,  and  of  the  same  number  of  ampere  turns ;  so 
that  the  armature  ampere  turns  and  the  ampere  turns  of  the 
compensating  winding  neutralize  each  other,  and  the  armature 


i8o  GENERAL  LECTURES 

reaction,  that  is,  the  magnetic  flux  produced  by  the  armature 
current,  and  the  self-induction  caused  by  it,  disappear. 

This  compensating  winding  for  neutralizing  the  armature 
self-induction  was  introduced  by  R.  Eickemeyer  in  the  early 
days  of  the  alternating  current  commutator  motor,  and  since 
then  all  alternating  current  commutator  motors  have  it ;  so  that 
the  electric  circuits  of  all  alternating  current  commutator 
motors  comprise  an  armature  winding  A,  a  field  winding  F, 
and  a  compensating  winding  C. 

Since  the  compensating  winding  cannot  be  identically  at 
the  same  place  as  the  armature  winding  (the  one  being  located 
in  slots  in  the  pole  faces,  the  other  in  slots  in  the  armature 
face)  there  still  exists  a  small  magnetic  flux  produced  by  the 
armature  winding :  the  "leakage  flux",  analogous  to  the  leakage 
flux  of  the  induction  motor ;  and  the  number  of  armature  turns 
cannot  be  increased  indefinitely,  otherwise  the  armature  self- 
induction,  due  to  this  leakage  flux,  would  become  appreciable, 
and  the  power  factor  would  decrease  again.  The  minimum 
total  self-induction  of  the  motor  with  compensating  winding 
occurs  at  a  number  of  armature  turns  equal  to  3  to  5  times  the 
field  turns ;  at  this  proportion,  the  power  factor  is  already  very 
good  at  low  speeds,  and  the  motor  is  industrially  satisfactory 
in  this  regard. 

For  best  results,  that  is,  complete  compensation  and  there- 
fore zero  magnetic  field  of  armature  reaction,  it  is,  however, 
necessary  not  only  to  have  the  same  number  of  ampere  turns  in 
the  compensating  winding  as  on  the  armature,  but  also  to  have 
these  ampere  turns  distributed  in  the  same  manner  around  the 
circumference.  With  the  usual  armature  winding  this  is  not 
the  case,  but  the  armature  conductors  cover  the  whole  circum- 
ference; while  the  compensating  coil  conductors  cover  only 
the  pole  arc,  as  the  space  between  the  poles  is  taken  up  by  the 


CM 

£  ; 


1 82  GENERAL  LECTURES 

field  winding.  That  is,  the  magnetic  distribution  around  the 
armature  circumference  is  as  shown  developed  in  Fig.  42: 
the  field  gives  a  flat  topped  distribution,  the  armature  a 
peaked,  and  the  compensating  winding  has  a  small  flat  top  and 
with  the  total  ampere  turns  of  the  compensating  winding  equal 
to  those  of  the  armature,  the  compensating  winding  preponder- 
ates in  front  of  the  field  poles,  the  armature  between  the  field 
poles,  or  at  the  brushes,  and  there  is  thus  a  small  magnetic  field 
of  armature  reaction  remaining  at  the  brushes,  just  where  it 
is  objectionable  for  commutation. 

As  it  is  not  feasible  to  distribute  the  compensating  wind- 
ing over  the  whole  circumference  of  the  stator,  the  armature 
winding  is  arranged  so  that  its  ampere  turns  cover  only  the 
pole  arcs.  This  is  done  by  using  fractional  pitch  in  the  arma- 
ture; that  is,  the  spread  of  the  armature  coil  or  the  space 
between  its  two  conductors,  is  made,  not  equal  to  the  pitch  of 
the  pole,  as  shown  in  Fig.  43,  but  only  to  the  pitch  of  the  pole 
arcs,  as  shown  in  Fig.  44.  With  such  fractional  pitch  winding, 
the  currents  in  the  upper  and  the  lower  layer  of  the  armature 
conductors,  in  the  space  between  the  poles,  flow  in  opposite  dir- 
ections, and  so  neutralize,  leaving  only  that  par/t  of  the  armature 
winding  in  front  of  the  pole  arcs  as  magnetizing.  Hereby  the 
distribution  of  the  armature  ampere  turns  is  made  the  same  as 
that  of  the  compensating  winding,  and  so  complete  compensa- 
tion is  realized. 

The  compensating  winding  may  be  energized  by  the  main 
current,  and  so  connected  in  series  with  the  field  and  armature ; 
or  the  compensating  winding  may  be  short  circuited  upon 
itself,  and  so  energized  by  an  induced  current  acting  as  a 
secondary  of  a  transformer  to  the  armature  as  primary ;  and  as 
in  a  transformer,  primary  and  secondary  current  have  the  same 
number  of  ampere  turns  (practically)  and  flow  in  opposite 


ALTERNATING  CURRENT  MOTOR     183 

directions,  such  "inductive  compensation"  is  just  as  complete 
compensation  as  the  "conductive  compensation"  produced  by 
passing  the  main  current  through  the  compensating  winding. 


Fig.  43 

Vice  versa,  the  armature  may  be  short  circuited  and 
so  used  as  secondary  of  a  transformer,  with  the  compensating 
winding  acting  as  primary.  In  either  of  these  motor  types, 


Fig.  44 

which  comprise  primary  and  secondary  circuits,  that  is,  in 
which  armature  and  compensating  winding  are  not  connected 
directly  in  series,  but  inductively,  the  field  may  be  energized 


184  GENERAL  LECTURES 

by  the  primary  or  supply  current,  or  by  the  secondary  or 
induced  current.  In  such  a  motor  embodying  a  transformer 
feature,  instead  of  impressing  the  supply  voltage  upon  one 
circuit  as  primary,  while  the  other  is  closed  upon  itself  as 
secondary,  the  supply  voltage  may  be  divided  in  any  propor- 
tion between  primary  and  secondary. 

As  primary  and  secondary  current  of  a  transformer  are 
proportional  to  each  other,  it  is  immaterial,  regarding  the  varia- 
tion of  the  current  in  the  different  circuits  with  the  load  and 
speed,  whether  the  circuits  are  directly  in  series,  or  by  trans- 
formation; that  is,  all  these  motors  have  the  same  speed 
— torque — current  characteristics,  as  discussed  in  the  preceding 
lecture,  and  differ  only  in  secondary  effects,  mainly  regarding 
commutation. 

The  use  of  the  transformer  feature  also  permits,  without 
change  of  supply  voltage,  to  get  the  effect  of  a  changed  supply 
voltage,  or  a  changed  number  of  field  turns,  by  shifting  a  cir- 
cuit over  from  primary  to  secondary  or  vice  versa.  For  in- 
stance, if  the  armature  is  wound  with  half  as  many  turns,  that 
is,  for  half  the  voltage  and  twice  the  current,  as  the  compen- 
sating winding,  by  changing  the  fitdd  from  series  connection 
with  the  compensating  winding  to  series  connection  with  the 
armature,  the  current  in  the  field  and  thus  the  field  strength,  is 
doubled;  that  is,  the  same  effect  is  produced  as  would  be  by 
doubling  (the  number  of  field  turns. 

According  to  the  relative  connection  of  the  three  circuits, 
armature  A,  compensating  circuit  C,  and  field  F,  alternating 
current  commutator  motors  of  the  series  type  can  be  divided 
into  the  classes  shown  diagramatically  in  Fig.  45 : 


10 

:          ^^-^ 


1  86  GENERAL  LECTURES 

Primary  :  Secondary  : 

A  +  C  +  F  Conductively     Compensated 

Series  Motor.  (2). 
A  +  F  C  Inductively      Compensating 

Series  Motor.  (3). 
A  C  +  F  Inductively      Compensating 

Series  Motor  with  Second- 

ary    Excitation,     or     In- 

verted   Repulsion   Motor. 

(4). 

C  +  F  A  Repulsion  Motor.  (5). 

C  A  +  F  Repulsion     Motor    with 

Secondary  Excitation  (6). 
C  &  A  +  F  Series  Repulsion  Motor  A. 


C  +  F  &  A  Series    Repulsion    Motor    B. 

(8). 

The  main  difference  between  these  types  of  motors  is 
found  in  .their  commutation. 

In  a  direct  current  motor,  with  the  brushes  set  at  the 
neutral;  that  is,  midway  between  the  field  poles  (as  is  custom- 
ary in  a  reversible  motor  like  the  series  motor),  the  armature 
turn,  which  is  shorted  circuited  under  the  brush  during  the 
commutation,  encloses  all  the  lines  of  magnetic  force  of  the 
field;  so  during  this  moment  it  does  not  cut  any  lines  of 
force  by  its  rotation,  and  thus  no  e.  m.  f  .  is  induced  in  this  turn  ; 
that  is,  no  current  is  produced,  if  the  armature  reaction  is  com- 
pensated for,  or  is  otherwise  negligible.  If  the  motor  has  a 
considerable  armature  reaction,  and  thus  a  magnetic  field  at  the 
brushes,  this  magnetic  field  of  armature  reaction  induces  an 
e.  m.  f.  in  the  short  circuited  turn  under  the  brush,  and  so 


ALTERNATING  CURRENT  MOTOR     187 

causes  sparking.     Hence  high  armature  reaction  impairs  the 
commutation  of  the  motor. 

In  an  alternating  current  series  motor  the  armature  reac- 
tion is  neutralized  by  the  compensating  winding,  and  therefore 
no  magnetic  field  of  armature  reaction  exists ;  hence  no  e.  m.  f . 
is  induced  in  the  turn  short  circuited  under  the  brush  by  its 
rotation  through  the  magnetic  field.  As  this  field,  however, 
is  alternating,  an  e.  m.  f.  is  induced  in  the  short  circuited  (turn 
by  the  alternations  of  the  lines  of  magnetic  force  enclosed  by 
it,  and  causes  a  short  circuit  current  and  in  that  way,  sparking. 
This  e.  m.  f.,  being  due  to  the  alternation  of  the  enclosed  field 
flux,  is  independent  of  the  speed  of  rotation ;  it  also  exists  with 
the  motor  at  a  standstill,  and  is  a  maximum  in  the  armature 
turn  under  the  brush,  as  this  encloses  the  total  field  flux.  The 
position  of  the  armature  turn  during  commutation,  which  in  a 
direct  current  motor  is  the  position  of  zero  induced  e.  m.  f., 
is  therefore  in  an  alternating  current  motor,  the  position  of 
maximum  induced  e.  m.  f.,  but  induced  not  by  the  rotation  of 
the  turn,  but  by  the  alternation  of  the  magnetism.  That  is,  this 
turn  is  in  the  position  of  a  short  circuited  secondary  to  the  field 
coil  of  the  motor  as  primary  of  a  transformer ;  and  as  primary 
and  secondary  ampere  turns  in  a  transformer  are  approximately 
equal,  the  current  in  -the  armature  turn  during  commutation 
is  very  large;  if  not  limited  by  the  resistance  or  reactance  of 
the  coil,  it  is  as  many  times  greater  than  the  full  load  current, 
as  the  field  coil  has  turns.  This  causes  serious  sparking,  if 
not  taken  care  of. 

One  way  of  mitigating  the  effect  of  this  short  circuit  cur- 
rent is  to  reduce  it  by  interposing  resistance  or  reactance ;  that 
is  by  making  the  leads  between  the  armature  turns  and  the 
commutator  bars  of  high  resistance  or  high  reactance.  Obvi- 
ously this  arrangement  can  merely  somewhat  reduce  the  spark- 


1 88  GENERAL  LECTURES 

ing  by  reducing  the  current  in  the  short  circuited  coil,  but  can 
not  eliminate  it ;  and  it  has  the  disadvantage,  that  in  the  moment 
of  starting,  if  the  motor  does  not  start  at  once,  the  resistance 
lead  is  liable  to  be  burned  out  by  excessive  heating ;  while  when 
running,  each  lead  is  in  circuit  only  a  very  small  part  of  the 
time:  during  the  moment  when  the  armature  turn  to  which 
it  connects,  is  under  a  commutator  brush.  As  the  resistance 
of  the  lead  must  be  very  much  greater  than  that  of  the  arma- 
ture coil,  and  as  the  space  available  for  it  is  very  much  smaller, 
if  remaining  in  circuit  for  any  length  of  time,  it  is  destroyed 
by  heat. 

In  direct  current  motors,  commutation  may  be  controlled 
by  an  interpole  or  commutating  pole;  that  is,  by  producing  a 
magnetic  field  at  the  brush,  in  direction  opposite  to  the  field 
of  armature  reaction,  and  by  this  field  inducing  in  the  arma- 
ture turn  during  commutation,  an  e.  m.  f.  of  rotation  which 
reverses  the  current.  Such  a  commutating  pole,  connected  in 
series  into  a  circuit,  would,  in  the  alternating  current  motor, 
induce  an  e.  m.  f.  in  the  short  circuited  turn,  by  its  rotation; 
but  this  e.  m.  f .  would  be  in  phase  with  the  field  of  the  commu- 
tating pole,  and  thus  with  the  current,  that  is,  with  the  main 
field  of  the  motor.  Therefore  it  could  not  neutralize  the  e.  m. 
f.  induced  in  the  short  circuited  turn  by  the  alternation  of  the 
main  field  through  it,  since  this  latter  e.  m.  f.  is  in  quadrature 
with  the  main  field,  and  thus  with  the  current;  but  would 
simply  add  itself  to  it,  and  so  make  the  sparking  worse.  A 
series  commutating  pole,  while  effective  in  a  direct  current 
motor,  is  therefore  ineffective  in  an  alternating  current  motor, 
due  to  its  wrong  phase. 

To  neutralize  the  e.  m.  f.  induced  by  the  alternation  of 
the  main  field  through  the  armature  turn  during  commutation, 
by  an  opposite  e.  m.  f.  induced  in  this  turn  by  its  rotation 


ALTERNATING  CURRENT  MOTOR     189 

through  a  quadrature  field  or  commutating  field,  this  field  must 
therefore  have  the  proper  phase.  The  e.  m.  f .  of  alternation  of 
the  main  field  through  the  short  circuited  turn  is  proportional  to 
the  main  field  F  and  frequency  N,  and  is  in  quadrature  with  the 
main  field.  The  e.  m.  f.  induced  in  the  short  circuited  turn  by 
its  rotation  (through  the  commutating  field  is  proportional  to 
the  frequency  of  rotation  or  speed  N0,  and  to  the  commutating 
field  F0,  and  in  phase  therewith ;  to  be  in  opposition  and  equal  to 
the  e.  m.  f .  of  alternation,  the  commutating  field ;  must  there- 
fore be  in  quadrature  with  the  main  field,  and  frequency  times 
main  field  must  equal  speed  times  commutating  field.  That  is : 

N  F  =  No  Fo 
or  in  other  words,  the  commutating  field  must  be : 

N 
F0  =  — F 

No 

or  equal  to  the  main  field  times  the  ratio  of  frequency  to  speed, 
and  in  quadrature  therewith. 

Hence,  at  synchronism:  N0  =  N,  the  commutating  field 
must  be  equal  to  the  main  field ;  at  half  synchronism : 

I 

NO  =  —  N,  it  must  be  twice ;  at  double  synchronism : 

2 

NO  =  2  N,  it  must  be  one-half  the  main  field. 

The  problem  of  controlling  the  commutation  of  the  alter- 
nating current  motor  therefore  requires  the  production  of  a 
commutating  field  of  proper  strength,  in  quadrature  phase  with 
the  main  field  of  the  motor,  and  thus  with  the  current. 

In  a  transformer,  on  non-inductive  or  nearly  non-induc- 
tive secondary  load,  the  magnetism  is  approximately  in  quad- 
rature behind  the  primary,  and  ahead  of  the  secondary  current ; 
transformation  between  compensating  winding  and  arma- 


1 90  GENERAL  LECTURES 

ture  thus  offers  a  means  of  producing  a  quadrature  field  in  the 
alternating  current  motor  for  compensation. 

In  the  conductively  compensated  series  motor,  at  perfect 
compensation,  no  quadrature  field  exists;  while  with  over 
or  under  compensation,  a  quadrature  field  exists,  in  phase  with 
the  current,  and  therefore  not  effective  as  commutating  field. 

In  the  inductively  compensated  series  motor,  the  quad- 
rature field,  which  transforms  current  from  the  armature  to 
the  compensating  winding,  is  of  negligible  intensity,  as  the 
compensating  winding  is  short  circuited,  and  thus  consumes 
very  little  voltage. 

A  quadrature  field,  however,  appears  in  those  motors  in 
which  the  compensating  winding  is  primary,  and  the  armature 
secondary,  that  is  in  repulsion  motors ;  since  in  the  armature  the 
induced  or  transformer  e.  m.  f.  is  opposed  by  the  e.  m.  f.  of 
rotation;  so  a  considerable  e.  m.  f.  is  induced,  and  therefore  a 
considerable  transformer  flux  exists. 

Therefore,  when  impressing  the  supply  voltage  on  the 
compensating  winding,  and  short  circuiting  the  armature  upon 
itself,  that  is,  in  the  repulsion  motor,  the  voltage  is  supplied  to 
the  armature  by  transformation  from  the  compensating  wind- 
ing, and  the  magnetic  flux  of  this  transformer  is  in  quadrature 
with  the  supply  current ;  that  is,  it  has  the  proper  phase  as  com- 
mutating flux. 

The  repulsion  motor  thus  has  in  addition  to  the  main  field, 
in  phase  with  the  current,  a  transformer  field,  in  quadrature 
with  the  main  field  in  space  and  in  time,  and  so  in  the  proper 
direction  and  phase  as  commutating  field ;  thus  giving  perfect 
commutation  if  this  transformer  field  has  the  intensity  required 
for  commutation,  as  discussed  above. 

As  in  the  repulsion  motor,  the  armature  is  short  circuited 
upon  itself,  the  voltage  supplied  to  it  by  transformation  from 


ALTERNATING  CURRENT  MOTOR     191 

the  compensating  winding  equals  the  voltage  consumed  in  it 
by  the  rotation  through  the  main  field.  The  former  voltage  is 
proportional  to  the  frequency  N  and  to  the  transformer  field  F1, 
the  latter  to  the  speed  N0  and  to  the  main  field  F,  and  it  so  is : 

N  F1  =  No  F, 
that  is,  the  transformer  field  is : 

No 
F1  =  _F 

N 

or  equal  to  the  main  field  times  the  ratio  of  speed  to  frequency. 
Comparing  this  value  of  the  transformer  field  of  the  repul- 
sion motor,  F1,  with  the  required  commutating  field  F0,  it  is 
seen  that  at  synchronism  N0  =  N,  F1  —  F0 ;  that  is,  the  trans- 
former field  of  the  repulsion  motor  has  the  proper  value  as 
commutating  field,  so  that  no  short  circuit  current  is  produced 
in  the  armature  turn  under  the  brush,  but  the  commutation  is 
as  good  as  in  a  direct  current  motor  with  negligible  armature 
reaction. 

i 
At  half  synchronism,  N0  =  —  N,  the  transformer  field  of 

2 

I 

the  repulsion  motor :  F1  =  —  F,  is  only  one  quarter  as  large  as 

2 

the  commutating  field  required  F0  =  2  F,  and  the  short  cir- 
cuit current  is  reduced  by  25%  below  the  value  which  it  has 
in  the  series  motor ;  and  the  commutation,  while  it  is  better,  is 
not  yet  perfect 

At  double  synchronism :  N0  =  2  N,  the  transformer  field 
is    F1  =  2    F,    while    the    commutation    field    should    be: 

i 

F0  =  —  F,  and  the  transformer  field  thus  is  four  times  larger 

2 
than  it  should  be  for  commutation ;  so  that  only  one-quarter  of 


i92  GENERAL  LECTURES 

the  transformer  field  is  used  to  neutralize  the  e.  m.  f.  of  alter- 
nation in  the  short  circuited  turn;  the  other  three-quarters 
induces  an  e.  m.  f.,  thus  causing  a  short  circuit  current  three 
times  as  large  as  it  would  be  in  a  series  motor.  That  is,  the 
short  circuit  current  under  the  brush,  and  thus  the  sparking,  in 
the  repulsion  motor  at  double  synchronism  is  very  much  worse 
than  in  the  series  motor,  and  the  repulsion  motor  at  these  high 
speeds  is  practically  inoperative. 

Hence,  as  regards  commutation,  a  repulsion  motor  is 
equal  to  the  series  motor  at  standstill  where  no  compensation 
of  (the  short  circuit  current  is  possible — but  becomes  better  with 
increasing  speed :  as  good  as  a  good  direct  current  series  motor 
at  synchronism;  and  then  again  becomes  worse  by  over  com- 
pensation, until  at  some  speed,  at  40%  above  synchronism,  it 
again  becomes  as  poor  as  the  alternating  current  series  motor ; 
above  this  speed,  it  becomes  rapidly  inferior  -to  the  series 
motor. 

To  produce  right  intensity  of  the  transformer  field,  to  act 
as  commutating  field,  it  is  therefore  necessary  above  syn- 
chronism to  reduce  the  transformer  field  below  the  value  which 
it  would  have  when  transforming  the  total  supply  voltage  from 
compensating  winding  to  armature.  This  means,  that  above 
synchronism,  only  a  part  of  the  supply  voltage  must  be  trans- 
formed from!  compensating  winding  to  armature,  the  rest 
directly  impressed  upon  the  armature.  Thus  at  double  syn- 
chronism, where  the  transformer  field  of  the  repulsion  motor 
is  four  times  as  strong  as  is  required  for  commutation,  to  re- 
duce it  to  one-quarter,  only  one-quarter  of  the  supply  voltage 
must  be  impressed  upon  the  compensating  winding,  three- 
quarters  directly  on  the  armature. 

To  get  zero  short  circuit  current  in  the  armature  turn 
under  the  brush,  below  synchronism  more  than  the  full  supply 


ALTERNATING  CURRENT  MOTOR     193 

voltage  would  have  to  be  impressed  upon  the  compensating 
winding,  which  usually  cannot  conveniently  be  done.  At  syn- 
chronism the  full  supply  voltage  is  impressed  upon  the  com- 
pensating winding,  while  the  armature  is  short  circuited  as 
repulsion  motor ;  and  with  increasing  speed  above  synchronism, 
more  and  more  of  the  supply  voltage  is  shifted  over  from  com- 
pensating winding  to  armature;  that  is,  the  voltage  impressed 
upon  the  compensating  winding  is  reduced,  from  full  voltage  at 
synchronism,  while  the  voltage  impressed  upon  the  armature 
is  increased,  from  zero  at  synchronism,  to  about  three-quarters 
of  the  supply  voltage  at  double  synchronism.  Such  a  motor, 
in  which  the  transformer  field  is  varied  in  accordance  with  the 
requirement  of  commutation,  is  called  a  "series  repulsion 
motor." 

The  arrangement  described  here  eliminates  the  short  cir- 
cuit current  induced  in  the  commutated  armature  turn  by  the 
alternation  of  the  main  field,  and  that  completely  above  syn- 
chronism, so  that  during  commutation,  no  current  is  induced 
in  the  armature  turn.  This,  however,  is  not  sufficient  for  per- 
fect commutation:  during  the  passage  of  the  armature  turn 
under  the  brush,  the  current  in  the  turn  should  reverse ;  so  that 
in  the  moment  in  which  the  turn  leaves  the  brush,  the  current 
has  already  reversed.  For  sparkless  commutation,  it  therefore 
is  necessary,  in  addition  to  the  neutralizing  e.  m.  f .  of  the  trans- 
former field,  to  induce  an  e.  m.  f.  which  reverses  the  current. 
This  e.  m.  f.,  and  thus  the  magnetic  flux  which  induces  it  by  the 
rotation,  must  be  in  phase  with  the  current.  That  is,  in  addi- 
tion to  the  "neutralizing"  component  of  the  commutating  field 
(which  is  in  quadrature  with  the  current),  to  reverse  the  cur- 
rent, a  second  component  of  the  commutating  field  must  exist, 
in  phase  with  the  current;  this  component  so  may  be  called 
the  "reversing  field".  The  total  commutating  field  required 


i94  GENERAL  LECTURES 

to  eliminate  the  short  circuit  current  due  to  the  alternating 
main  field  by  the  "neutralizing"  flux,  and  to  reverse  the  arma- 
ture current  by  the  "reversing  flux",  must  therefore  be  some- 
what less  than  90°  lagging  behind  the  main  field  and  thus  the 
main  current. 

While  in  a  transformer  with  non-inductive  load  on  the 
secondary,  the  magnetic  flux  lags  nearly  90°  behind  the 
primary  current,  in  a  transformer  with  inductive  load  on  (the 
secondary,  the  magnetic  flux  lags  less  than  90°  behind  the 
primary  current;  and  the  more  so  the  higher  the  inductivity  of 
the  secondary  load. 

Therefore,  by  putting  a  reactance  into  the  armature  circuit 
of  the  motor,  and  so  making  the  armature  circuit  inductive, 
the  transformer  flux  is  made  to  lag  less  than  90°  behind  the 
current,  and  act  not  only  as  neutralizing  but  also  as  reversing 
flux;  and  so,  if  it  be  of  proper  intensity,  it  gives  perfect  commu- 
tation. 

An  additional  reactance  would  in  general  be  objectional, 
in  lowering  the  power  factor  of  the  motor.  The  motor,  how- 
ever, contains  a  reactance:  its  field  circuit,  which  has  to  be 
excited,  can  be  used  as  reactance  for  the  armature  circuit. 
That  is,  by  connecting  the  field  coils  into  the  armature  cir- 
cuit, or  in  other  words,  using  secondary  excitation,  the  trans- 
former flux  of  .the  motor  is  given  the  lead  ahead  of  quad- 
rature position  with  the  main  field,  which  is  required  to  act  as 
reversing  field. 

In  this  manner,  it  is  possible  in  the  alternating  current 
commutator  motor,  to  get  at  all  speeds  from  synchronism 
upwards,  the  same  perfect  commutation  as  in  a  direct  current 
motor  with  commutating  poles,  by  varying  the  distribution  of 
supply  voltage  between  compensating  winding  and  armature, 
and  exciting  the  field  in  series  with  the  armature  circuit ;  that 


196  GENERAL  LECTURES 

is,  in  the  series  repulsion  motor  B  of  the  preceding  table. 
Obviously,  this  distribution  of  voltage  would  for  all  practical 
purposes  be  carried  out  sufficiently  by  using  a  number  of  steps, 
as  shown  diagrammatically  by  the  arrangement  in  Fig.  46 : 

T  is  the  supply  circuit,  F  the  field  winding,  A  the  arma- 
ture, and  C  the  compensating  winding. 

Closing  switch  i,  and  leaving  all  others  open,  the  motor 
is  a  repulsion  motor. 

Closing  switch  2,  and  leaving  I  and  all  others  open,  the 
motor  is  a  repulsion  motor  with  secondary  excitation. 

Closing  switch  3,  or  4,  5  .  .  .  and  leaving  all  others 
open,  the  motor  is  a  series  repulsion  motor  B,  with  gradually 
increasing  armature  voltage  and  decreasing  voltage  on  the 
compensating  winding. 

By  winding  the  armature  for  half  the  voltage  and  twice 
the  current  of  the  compensating  winding,  when  changing  from 
position  i,  the  field  in  the  compensating  circuit,  to  the  next 
position,  with  the  field  in  the  armature  circuit,  (the  field  current 
and  the  field  strength  becomes  double  the  value  it  had  in  start- 
ing, where  no  compensation  exists,  and  which  it  would  have 
to  maintain  in  a  series  motor;  and  thus  a  correspondingly 
greater  motor  output  is  secured,  than  would  be  possible  in  a 
motor  in  which  the  commutation  is  not  controlled. 


FIFTEENTH  LECTURE 


ELECTROCHEMISTRY 

LECTROCHEMISTRY  is  one  of  the  most  important 
applications  of  electric  power,  and  possibly  even  more 
power  is  used  for  electrochemical  work  than  for  rail- 
roading. 

In  electrochemical  industries  the  most  expensive  part  is 
electric  power;  material  and  labor  are  usually  much  less. 
Such  industries  therefore  are  located  at  water  powers,  where 
the  cost  of  power  is  very  low. 

The  main  classes  of  electrochemical  work  are : 

A.  Electrolytic. 

B.  Electrometallurgical. 

A.     ELECTROLYTIC  WORK. 

The  chemical  action  of  the  current  is  used,  by  electrolyz- 
ing  either  solutions  of  salts  or  fused  salts  or  compounds. 

Electrolysis  of  solutions  in  water  is  possible  only  with 
such  metals  which  have  less  chemical  affinity  than  hydrogen. 
For  instance,  Cu,  Fe,  and  Zn  can  be  deposited  from  salt  solu- 
tions in  water,  but  not  Al,  Mg,  Na,  etc.  Electrolyzing,  for 
instance,  NaCl  (salt  solution)  the  sodium  (Na)  which  appears 
at  the  negative  terminal  immediately  dissociates  the  water  and 
gives  Na  +  H2O  =  NaOH  +  H,  or:  sodium  plus  water  = 
caustic  soda  plus  hydrogen. 

It  takes  1.4  volts  to  electrolyze  water;  any  metal 
requiring  more  than  1.4  volts  for  separation  therefore  is  not 
separated,  but  hydrogen  is  produced. 

Therefore  the  highest  voltage  used  is  an  electrolytic  cell 
containing  water  is  1.4  +  the  ir  drop  in  the  resistance  of  the 


200  GENERAL  LECTURES 

cell;  which  latter,  for  reasons  of  economy,  is  made  as  low  as 
possible. 

Even  fused  salts  require  fairly  low  voltage,  at  the  highest 
from  3  to  4  volts. 

Since  the  voltage  required  per  cell  is  very  low,  a  large 
number  of  cells  are  connected  in  series,  and  even  then  large 
low  voltage  machines  are  required. 

Some  of  the  important  applications  of  electrolysis  are : 
Electroplating;   that   is,    covering   with   copper,    nickel, 
silver,  gold,  etc. 

Electro  typing;  that  is,  making  of  copies,  usually  of  cop- 
per; and  especially 

Metal  refining. 

A  very  large  part  of  all  the  copper  used  is  electrically 
refined.  The  crude  copper  as  cast  plate  is  used  as  anode  or 
positive,  and  a  thin  plate  of  refined  copper  is  used  as  cathode,  or 
negative  (terminal  in  a  copper  sulphate  solution.  The  anode  is 
dissolved  by  the  current  and  the  fine  copper  is  deposited  on  the 
cathode ;  while  silver  and  gold  go  down  into  the  mud,  lead  goes 
into  the  mud  as  sulphate,  tin  as  oxide;  sulphur,  selenium  and 
tellurium,  arsenic  and  other  impurities  also  go  in  the  mud ;  and 
zinc  and  iron  remain  in  solution  as  sulphates  if  the  current 
density  is  sufficiently  low.  If  the  current  density  is  high,  some 
zinc  and  iron  may  deposit :  zinc  and  iron  have  a  greater  chemi- 
cal activity  than  copper,  since  they  precipitate  copper  from 
solution.  Therefore  it  takes  more  power,  that  is,  more  voltage, 
to  deposit  zinc  and  iron,  than  it  takes  to  deposit  copper.  If  the 
current  density  is  low,  the  voltage  required  to  deposit  the 
copper  plus  the  ir  drop,  that  is,  the  total  voltage  of  the  cell,  is 
less  than  the  voltage  required  to  deposit  zinc  or  iron,  and  they 
do  not  deposit,  but  dissolve  at  the  anode  and  remain  in  solution. 


ELECTROCHEMISTRY  201 

At  higher  current  density  the  ir  drop  in  the  cell  is  higher ; 
thus  the  total  voltage  of  the  cell  is  higher,  and  may  become 
high  enough  to  deposit  iron  or  even  zinc. 

If  the  anode  is  crude  copper,  the  cathode  pure  copper, 
the  voltage  at  the  anode  is  higher  than  at  the  cathode  and  the 
cell  takes  some  voltage.  The  voltage  required  for  copper 
refining  is  the  higher,  the  more  impure  the  copper  is;  but  is 
always  very  low,  usually  a  fraction  of  a  volt,  and  therefore 
very  many  cells  are  run  in  series. 

The  solution  gradually  becomes  impure  and  has  to  be 
replaced. 

Other  metals  are  occasionally  refined  electrolytically,  but 
only  to  a  small  extent. 

Metal  Reduction. 

Metals  are  reduced  from  their  ores  electrolytically, 
especially  such  metals  which  have  so  high  chemical  affinity  that 
they  are  not  reduced  by  heating  with  carbon.  In  this  way 
aluminum,  magnesium,  sodium,  calcium,  etc.,  are  made  electro- 
lytically. Since  their  chemical  affinity  is  greater  than  that  of 
hydrogen,  they  cannot  be  deposited  from  solutions  in  water, 
but  only  from  fused  salts,  or  solutions  in  fused  salts.  So  cal- 
cium is  produced  now  by  electrolyzing  fused  calcium  chloride, 
CaCl2.  Aluminum  is  made  by  electrolyzing  a  solution  of 
alumina  in  melted  cryolithe  (sodium  aluminum  fluoride). 

SECONDARY  PRODUCTS. 

Frequently  electrolysis  is  used  to  produce  not  the  sub- 
stances which  are  directly  deposited,  but  substances  produced 
by  the  reaction  of  these  deposits  on  the  solutions.  For  instance, 
electrolyzing  a  solution  of  salt,  NaCl,  in  water,  we  get  sodium, 
Na,  at  the  negative,  chlorine,  Cl,  at  the  positive  terminal. 


202  GENERAL  LECTURES 

If  we  use  mercury,  Hg,  as  negative  electrode,  it  dissolves 
the  sodium  and  so  we  get  sodium  amalgam. 

Otherwise  the  sodium  dees  not  deposit  but  immediately 
acts  upon  the  water  and  forms  sodium  hydrate  or  caustic  soda, 
NaOH. 

The  chlorine,  Cl,  at  the  anode  also  reacts  on  the  water,  one 
chlorine  atom  taking  up  one  hydrogen  and  another  chlorine 
atom  the  remaining  OH  of  the  water  H2O;  that  is,  we  get 
2C1  +  H2O  =  C1H  +  C1OH,  that  is,  hydrochloric  +  hypo- 
chlorous  acid. 

With  the  sodium  hydrate  from  the  other  cathode  these 
acids  form  NaCl  and  ClONa,  that  is  sodium  chloride  and 
hypochlorite,  or  bleaching  soda. 

If  the  solution  is  hot,  the  reaction  goes  further  and  we  get 
6C1  +  3H2O  =  5C1H  +C1O3H,  that  is  hydrochloric  and 
chloric  acid,  and  with  the  sodium  hydrate  from  the  other  side 
(these  form  NaCl  and  ClO3Na,  that  is,  sodium  chloride  and 
sodium  chlorate. 

In  this  way  considerable  industries  have  developed,  pro- 
ducing electrolytically  caustic  soda,  bleaching  soda,  and 
chlorates. 

Alternating  current  is  used  very  little  for  electrolytic 
work,  as  with  organic  compounds  to  produce  oxidation  and 
reduction  at  the  same  time;  that  is,  act  on  the  compound  in 
rapid  succession  by  oxygen  and  hydrogen,  the  one  during  the 
one,  the  other  during  the  next  half  wave  of  current. 

Very  active  metals  like  manganese  and  silicon  dissolve  by 
alternating  current;  that  is,  one-half  wave  dissolves,  but  the 
other  does  not  deposit  again. 

Very  inert  metals  like  platinum  are  deposited  by  alternat- 
ing current;  that  is,  the  negative  half  wave  deposits  by  alter- 
nating current,  but  the  positive  half  wave  does  not  dissolve. 


ELECTROCHEMISTRY  203 

B.     ELECTROMETALLURGICAL  WORK. 

In  electrometallurgical  work  the  heat  is  used  to  produce 
the  chemical  action;  thus  it  is  immaterial  whether  alternating 
or  direct  current  is  used. 

The  voltage  required  is  still  low  but  not  as  low  as  in  elec- 
trolytic work : 

The  carborundum  furnace  takes  from  250  to  90,  mostly 
about  loo  volts;  that  is,  it  starts  cold  with  250  volts.  While 
heating  up  the  resistance  drops,  and  the  voltage  decreases 
down  to  100  volts  when  the  furnace  is  hot  and  remains  there 
until  towards  the  end.  Then  the  inner  layer  of  carborundum 
begins  to  change  to  graphite  and  the  resistance,  and  therefore 
the  voltage  falls. 

The  carbide  furnace  and  arc  furnaces  in  general  take 
from  50  to  100  volts ;  the  graphite  furnace  takes  from  10  to  20 
volts. 

To  get  very  high  temperatures  a  very  large  amount  of 
energy  has  to  be  concentrated  in  one  furnace;  and  with  the 
moderate  voltage  used,  this  requires  very  large  currents, 
thousands  of  amperes.  Alternating  currents  are  almost  exclu- 
sively used,  since  it  is  easier  to  produce  very  large  alternating 
currents  by  transformers,  and  since  it  is  easier  to  control  alter- 
nating than  direct  currents. 

Electric  heat  necessarily  is  very  much  more  expensive  than 
heat  produced  by  burning  coal,  and  so  the  electric  furnace 
is  used  mainly: 

ist.  Where  very  perfect  control  of  the  temperatures  and 
freedom  from  impurities  is  essential. 

2nd.  Where  temperatures  higher  than  can  be  produced 
by  combustion  are  required. 

i.  Very  accurate  temperature  regulation  and  freedom 
from  impurities,  for  instance,  are  important  in  making  and 


204  GENERAL   LECTURES 

annealing  high  grade  tool  steels,  etc.  By  using  coal  or  oil  as 
fuel,  contamination  by  the  gases  of  combustion,  and  by  the 
metal  taking  up  carbon  or  (if  an  excess  of  air  is  used,  oxygen) 
is  difficult  to  avoid. 

By  electric  heating,  by  resistance  at  lower  temperature 
and  by  induction  furnace  at  higher  temperature,  contamination 
can  be  perfectly  avoided  and  even  the  air  can  be  excluded. 

2.     The  temperature  of  combustion  is  limited. 

Four-fifths  of  air  is  nitrogen  which  does  not  take  part  in 
the  combustion,  but  which  has  to  be  heated,  thus  greatly  lower- 
ing the  temperature;  therefore  combustion  in  air,  even  if  the 
air  is  preheated,  gives  a  lower  temperature  than  when  using 
oxygen.  But  even  the  temperature  of  the  oxy-hydrogen,  or 
the  oxy-acetylene  flame  is  only  just  able  to  melt  platinum. 

The  temperature  which  can  be  reached  by  combustion,  is 
limited,  since  at  very  high  temperature  the  chemical  affinity 
of  oxygen  for  hydrogen  and  carbon  ceases :  water  dissociates, 
that  is,  spontaneously  splits  up  in  hydrogen  and  oxygen  at 
2000  degrees  Centigrade  and  no  temperature  higher  than 
2000°  can  therefore  be  reached  by  the  oxy-hydrogen  flame; 
carbon  dioxide,  CO2,  already  dissociates  at  about  I5OO°C  into 
carbon  monoxide,  CO,  and  oxygen,  O.  Carbon  monoxide,  CO, 
splits  up  into  carbon  and  oxygen  not  much  above  2000° C.  (In 
all  high  temperature  reactions  of  carbon,  as  in  the  formation 
of  carbides,  CO  therefore  always  forms  and  not  CO2,  since 
CO2  cannot  exist  at  a  very  high  temperature ;  and  1he  CO  when 
leaving  the  furnace  then  burns  to  CO2  with  blue  flame) . 

Higher  temperatures  than  those  generated  by  the  com- 
bustion of  carbon  and  hydrogen  can  be  produced  by  the  com- 
bustion of  those  elements  whose  oxides  are  stable  at  very  high 
temperatures ,  as  aluminum  and  calcium.  In  this  way,  many 
metals,  as  chromium  and  manganese,  which  cannot  be  reduced 


ELECTROCHEMISTRY  205 

from  the  oxides  by  carbon  (due  to  the  lower  temperature  of 
carbon  combustion)  can  be  reduced  by  aluminum  in  the  "ther- 
mite" process.  That  is,  their  oxides  are  mixed  with  powdered 
aluminum  and  then  ignited :  the  aluminum  burns  in  taking  up 
the  oxygen  of  the  metal,  and  so  produces  an  extremely  high 
temperature,  which  melts  the  metal  and  the  alumina  (corun- 
dum) which  is  produced. 

Since,  however,  all  the  aluminum  is  made  electrolytically, 
the  thermite  process  still  requires  the  use  of  electric  power. 
The  temperature  of  combustion  of  aluminum,  however,  is 
still  far  below  that  of  the  electric  carbon  arc,  since  in  the  car- 
bon arc,  alumina  boils. 

For  temperatures  above  2000°  to  25oo°C,  and  up  to  the 
arc  temperature  or  about  35OO°C,  electric  energy  is  therefore 
necessary. 

Electric  furnaces  are  of  two  classes : 

Arc  Furnaces  and  Resistance  Furnaces. 

In  the  resistance  furnace  any  temperature  can  be  produced 
up  to  the  point  of  destruction  of  the  resistance  material,  that 
is,  up  to  35OO°C,  when  using  carbon. 

The  arc  furnace  gives  the  arc  temperature  of  35oo°C,  but 
allows  the  concentration  of  much  more  energy  in  a  small  space 
and  thus  produces  reactions  requiring  the  very  highest  temper- 
atures. 

Some  of  the  electrometallurgical  industries  are : 

(a).  Calcium  carbide  production.  Arc  furnaces  are 
used  and  the  reaction  is 

CaO  +  3C  =  CaC2  +  CO. 

A  mixture  of  coke  and  quick  lime  is  used  in  the  process. 

(b).  Carborundum  production.  A  resistance  furnace  is 
used,  containing  a  carbon  core  about  24  feet  long,  around 
which  the  material  is  placed  and  heated  by  the  current  passing 


2o6  GENERAL  LECTURES 

through  the  core.    The  furnace  takes  1000  HP  and  the  reac- 
tion is : 

SiO2  +  3C  =  SiC  +  2CO. 

The  material  is  a  mixture  of  sand,  coke,  sawdust  and  salt. 

(c).  Graphite  furnace.  A  resistance  furnace  somewhat 
similar  to  the  carborundum  furnace  is  used,  but  with  lower 
voltage  and  larger  currents ;  the  material  is  coke  or  anthracite, 
which  by  the  high  temperature  is  converted  into  graphite, 
probably  passing  through  an  intermediate  stage  as  a  metal  car- 
bide. 

(d).  Silicon  furnace.  Either  arc  or  resistance  furnace 
is  used ;  the  reaction  is : 

Si02  +  2C  =  Si  +  2CO. 

or, 

Si02  +  2SiC  =  3Si  +  2CO. 

(e).     Titanium  carbide  furnace.     Arc  or  resistance  fur- 
nace is  used  which  requires  a  very  high  temperature;  that  is, 
a  greater  temperature  than  that  of  the  calcium  carbide  furnace. 
Ti02  +  3C  =  TiC  +  2CO. 

Other  products  of  the  electric  furnace  are  siloxicon,  sili- 
con monoxide,  etc.,  and  numerous  alloys  of  refractory  metals, 
mainly  with  iron;  as  of  vanadium,  tungsten,  molybdenum, 
titanium,  etc.,  which  are  used  in  steel  manufacture. 

The  use  of  the  electric  arc  for  the  production  of  nitric 
acid  and  nitrate  fertilizers ;  of  the  high  potential  glow  discharge 
for  the  production  of  ozone  for  water  purification,  etc.,  also 
are  applications  of  electric  power,  which  are  of  rapidly  increas- 
ing industrial  importance. 


SIXTEENTH  LECTURE 


THE  INCANDESCENT  LAMP 

HE  two  main  types  of  electric  illuminants  are  the  in- 
candescent lamp  and  the  arc. 

In  the  incandescent  lamp  the  current  flows  through 
a  solid  conductor,  usually  in  a  vacuum,  and  (the  heat  produced 
in  the  resistance  of  the  conductor  makes  it  incandescent,  thus 
giving  the  light.  Incandescent  lamps  in  an  electric  circuit 
therefore  act  as  non-inductive  ohmic  resistance  and  can  there- 
fore be  operated  equally  well  on  constant  potential  as  on  con- 
stant current.  As  electric  distribution  systems  are  always 
constant  potential,  most  incandescent  lamps  are  operated  on 
constant  potential ;  and  only  for  outdoor  lighting,  that  is,  for 
street  lighting  in  cases  where  the  arc  lamp  is  too  large  and  too 
expensive  a  unit  of  light  for  the  requirements,  incandescent 
lamps  are  used  on  a  constant,  direct  or  alternating  current  cir- 
cuit ;  they  are  then  usually  built  for  the  standard  arc  circuits, 
and  thus  for  low  voltage. 

For  general  convenience  the  efficiency  of  incandescent 
lamps  is  given  in  watts  power  consumption  per  horizontal 
candle  power,  when  operating  on  such  a  voltage,  that  the 
candle  power  of  the  lamp  decreases  by  20%  in  500  hours  run- 
ning; and  the  time,  in  which  the  candle  power  decreases  by 
20% — that  is,  500  hours  with  the  present  efficiency  rating — is 
called  the  useful  life ;  since  experience  has  shown,  that  after  a 
decrease  of  candle  power  of  20%,  with  the  carbon  filament 
lamp,  under  average  conditions,  it  is  more  economical  to 
replace  the  lamp  with  a  new  lamp,  than  to  continue  its  use ;  as 
then  the  increased  cost  of  light  due  to  'the  lower  efficiency  is 
greater  than  the  cost  of  the  lamp,  when  distributed  over  500 
hours. 


210  GENERAL  LECTURES 

In  discussing  incandescent  lamp  efficiencies,  it  is  therefore 
essential  to  make  sure  that  the  efficiency  is  given  at  the  useful 
life  of  500  hours;  since  obviously  any  efficiency  can  be  pro- 
duced in  any  lamp,  by  running  it  at  higher  voltage,  but  the  life 
is  greatly  shortened  thereby.  Therefore  efficiency  compari- 
sons have  a  meaning  only  when  based  on  the  same  length  of 
useful  life,  as  500  hours. 

Obviously,  for  other  types  of  lamps,  the  economic  life 
may  be  greater  (as  for  more  expensive  lamps)  or  less  than 
500  hours. 

Illummants  are  measured  and  compared  by  the  total  flux 
of  light  which  they  give.  Usually,  however,  this  is  expressed 
in  "mean  spherical  candle  power";  that  is,  the  candle  power 
which  would  be  given  by  the  illuminant  if  this  light  were  dis- 
tributed uniformly  throughout.  Since  the  object  of  a  lamp 
is  to  give  light,  obviously  the  only  logical  way  of  measuring  it 
is  by  the  total  amount  of  light  which  it  gives,  and  so  by  the 
mean  spherical  candle  power ;  this  therefore  is  standard. 

The  conventional  rating  of  the  incandescent  lamp,  in  hori- 
zontal candle  power,  therefore  has  to  be  multiplied  by  a  reduc- 
tion factor,  to  give  the  mean  spherical  candle  power.  With  the 
carbon  filament  lamp,  this  reduction  factor  is  usually  .79;  a 
1 6  candle  power  so  has  a  mean  spherical  candle  power  of 
16  x  .79  =  12.6  c.  p.,  and  at  an  efficiency  of  3.1  watts  per 

3-i 
horizontal  candle  power,  it  has  an  efficiency  of ==  3.92 

•79 

watts  per  mean  spherical  candle  power. 

The  carbonized  bamboo  fibre  used  in  the  very  early  days 
was  very  soon  replaced  by  filaments  made  of  structureless 
cellulose,  squirted  from  a  cellulose  solution,  and  then  carbon- 
ized. By  "treating"  these  filaments,  that  is,  heating  them  in 


THE  INCANDESCENT  LAMP  211 

gasolene  vapor  and  therefrom  depositing  a  thin  shell  of  car- 
bon on  them,  a  considerable  increase  in  efficiency  became  pos- 
sible; their  efficiency  was  thus  greatly  increased,  from  5  to  6 
watts  per  candle  power  in  the  early  days,  to  3.5  and  3.1  watts 
per  candle  power.  Of  these  two  types,  the  3.5  watt  lamp  is  used 
in  systems  of  poor  voltage  regulation,  in  which  (the  more 
efficient  3.1  watt  lamp  would  have  too  short  a  life;  with  the 
improvements  in  the  voltage  regulation  of  systems,  the  less 
efficient  3.5  watt  lamp  is  thus  coming  out  of  use. 

By  exposing  these  "treated"  filaments  to  the  highest 
temperature  of  the  electric  furnace,  their  stability  at  high 
temperature  is  greatly  improved ;  so  that  in  these  "metallized"* 
filament  lamps  an  efficiency  of  2.5  to  2.6  watts  per  candle 
power  is  reached.  Whether  a  still  further  increase  of  efficiency 
of  the  carbon  filament  will  occur,  as  is  quite  possible,  or 
whether  the  carbon  filament  will  be  replaced  by  the  metal  fila- 
ments, remains  for  the  future  to  decide. 

In  the  last  years,  metal  filament  lamps  giving  efficiencies 
far  higher  than  has  so  far  been  possible  to  reach  with  the  car- 
bon filament,  have  been  developed.  First  came  the  os- 
mium lamp,  of  1.5  watts  per  candle  power.  As  the  total 
supply  of  osmium  available  on  (the  earth  is  far  less  than 
would  be  required  for  one  year's  production  of  incandescent 
lamps,  the  osmium  lamp  never  could  hope  for  more  than  a 
very  limited  use.  The  tantalum  lamp,  which  was  developed 
next,  and  is  now  quite  extensively  used,  gives  an  efficiency  of 
about  2  watts  per  candle  power;  that  is,  it  is  not  quite  as 
efficient  as  -the  osmium  lamp,  since  tantalum  is  somewhat  more 
fusible  than  osmium.  As  tantalum  is  a  metal  which  can  be 
drawn  into  wire,  the  tantalum  filament  is  of  drawn  wire ;  while 

*  The  name  "metallized"  is  given  to  the  form  of  carbon  produced  in  these  filaments  by  the  elec- 
tric furnace  temperature,  since  it  has  metallic  resistance  characteristics-:  a  positive  temperature 
coefficient  of  resistance,  while  the  other  forms  of  carbon  have  a  negative  temperature  coefficient 


GENERAL  LECTURES 

all  the  other  metals  which  are  used  for  lamp  filaments  are  not 
ductile,  and  the  filaments  have  to  be  made  by  some  squirting 
process,  similar  to  the  carbon  filament.  The  highest  efficiency 
was  reached  by  the  tungsten  (wolfram)  lamp,  of  i  to  iV4 
watts  per  candle  power;  that  is,  tungsten  (or  rather  wolfram 
metal,  since  tungsten  is  the  name  of  the  ore  of  the  metal),  has 
the  highest  melting  point  of  all  known  metals,  and  so  can  be 
run  at  the  highest  temperature,  that  is,  highest  efficiency. 

All  these  metals  melt  far  below  the  temperature  where 
carbon  melts  or  boils,  but  carbon  has  the  great  disadvantage  of 
evaporating  considerably  below  its  melting  point,  while  these 
metals  evaporate  very  little,  and  so  can  be  run  at  a  temperature 
fairly  close  to  their  melting  point;  while  the  carbon  filament 
has  to  be  operated  at  a  temperature  very  far  below  the  melting 
point. 

The  great  difficulty  with  all  these  metal  filaments  is,  that 
the  metals  are  very  much  better  conductors  than  carbon; 
to  get  the  same  filament  resistance,  so  as  to  consume  the  same 
current,  at  the  same  voltage,  the  metal  filaments  must  be  very 
much  longer  and  very  much  thinner  than  the  carbon  filament. 
As  the  efficiency  of  the  metal  filament  is  far  higher,  *to  produce 
the  same  candle  power  at  the  same  voltage,  less  current  and 
therefore  a  higher  resistance  is  required,  which  makes  these 
metal  filaments  still  thinner ;  as  a  result,  although  the  metals  are 
mechanically  stronger  than  carbon,  the  metal  filaments  are  far 
more  frail,  due  to  their  exceeding  thinness,  and  it  is  very  diffi- 
cult to  produce  lamps  of  as  low  candle  power,  as  is  feasible  with 
carbon  filaments.  For  larger  units,  however,  and  for  larger 
current  low  voltage  lamps,  for  series  lighting,  the  metal  fila- 
ments are  specially  suited. 

For  general  use,  the  16  candle  power  lamp  has  proved  the 
moat  convenient  unit  of  light.  The  limitation  of  voltage,  for 


THE  INCANDESCENT  LAMP  213 

which  efficient  incandescent  lamps  of  such  size  can  be  built, 
has  been  the  cause  of  the  general  use  of  1 10  volt  distribution. 
220  volt  1 6  candle  power  carbon  filament  lamps  can  be  built, 
but  are  of  necessity  less  efficient,  by  about  15%,  than  no  volt 
lamps :  at  220  volts,  half  ithe  current  and  so  four  times  the 
resistance  is  required  for  the  same  power  as  at  no  volts;  the 
filament  therefore  is  about  twice  as  long  and  half  as  thick, 
hence  more  breakable  and  more  rapidly  disintegrating ;  so  that 
there  is  no  possibility  of  reaching  the  same  efficiency  in  a  220 
volt  1 6  candle  power  lamp,  as  in  no  volt  lamp  made  with  the 
same  care.  For  the  same  reason,  the  8  candle  power  no  volt 
lamp  must  be  less  efficient  than  the  16  candle  power  no  volt 
lamp. 

In  an  incandescent  lamp  are  specified :  the  candle  power, 
the  efficiency,  and  the  voltage.  To  produce  lamps  fulfilling 
simultaneously  all  three  conditions,  requires  either  to  allow  a 
large  margin  in  either  condition — that  is,  gives  a  product 
inferior  in  uniformity — or  to  get  a  uniform  product,  a  large 
percentage  is  thrown  out  as  defective,  and  the  cost  of  the  lamp 
is  thus  seriously  increased.  For  this  reason,  in  the  manufacture 
a  very  close  agreement  is  aimed  at  in  candle  power  and  in 
efficiency;  the  lamps  are  then  assorted  for  voltages,  and  dif- 
ferent voltages  are  then  assigned  by  the  organization  of  illum- 
inating companies  to  the  different  companies,  so  as  to  tonsume 
the  total  lamp  product.  As  a  result  hereof,  a  far  more  uniform 
product  is  derived  than  could  be  derived  in  any  other  way,  and 
than  is  available  in  any  other  country.  This  is  the  reason,  that 
in  distribution  systems  not  one  and  the  same  voltage,  as  no,  is 
employed  throughout ;  but  different  cities  use  different  voltages, 
between  105  and  130.  The  average  incandescent  lamp  used  in 
this  country  therefore  is  decidedly  superior  in  uniformity  and 
in  efficiency  to  those  used  abroad.  The  ultimate  cause  hereof 


2i4  GENERAL  LECTURES 

is,  that  since  the  earliest  days  the  illuminating  companies  have 
followed  the  principle  of  supplying  light,  and  not  power  ;*  and 
220  volt  distribution,  while  being  more  efficient  from  the  gen- 
erating station  to  the  customer's  meter,  is  decidedly  inferior  in 
efficiency  from  the  generating  station  to  the  candle  power  pro- 
duced at  the  customer's  lamps,  as  the  saving  in  distribution 
losses  does  not  make  up  for  the  lower  efficiency  of  the  220  volt 
lamp.  For  this  reason,  220  volt  distribution  has  never  found 
any  entrance  in  this  country. 

In  gas  lighting,  an  enormous  increase  of  efficiency 
resulted  from  the  development  of  the  Welsbach  gas  mantle. 
In  the  same  direction,  that  is,  by  using  what  may  be  called 
"heat  luminescence"  in  electric  lighting,  the  Nernst  lamp  was 
developed,  using  the  same  class  of  material :  refractory  metal- 
lic oxides,  as  in  the  Welsbach  mantle.  The  "glower"  of  the 
Nernst  lamp,  however,  is  a  non-conductor  at  ordinary  tempera- 
ture, and  requires  some  heating  device,  the  "heater",  to  be 
made  conducting.  When  conducting,  it  has  a  very  high  nega- 
tive temperature  coefficient;  that  is,  the  voltage  consumed  by 
the  glower  decreases  with  the  increase  of  current,  just  as  in 
the  arc,  and  it  therefore  requires  a  steadying  resistance,  called 
the  "ballast".  The  lamp  therefore  requires  some  operating 
mechanism,  to  cut  the  heater  out  of  circuit  after  the  glower  is 
started.  The  glower  of  the  Nernst  lamp  is  not  operative  in  a 
vacuum,  since  air  seems  to  be  necessary  for  its  heat  lumines- 
cence. Fairly  good  efficiencies  have  been  reached  with  these 
lamps,  especially  in  larger  units,  as  3  to  6  glower  lamps,  but 
not  of  the  same  class  as  with  the  tungsten  lamp. 

*  Fora  long  time,  the  bills  were  even  made  out  in 'Mamp   hoars."   and   in   the   earlier   dayi    the 
machines  rated  in  "lights"  and  not  in  kilowatts. 


SEVENTEENTH  LECTURE 


ARC  LIGHTING 

HILE  incandescent  lamps  can  be  operated  on  constant 
potential  as  well  as  on  constant  current,  the  arc  is 
essentially  a  constant  current  phenomenon.  At  con- 
stant length,  the  voltage  consumed  by  the  arc  decreases  with 
increase  of  current,  as  shown  by  curve  I  in  Fig.  47.  If,  there- 
fore, an  attempt  is  made  to  operate  such  an  arc  on  constant 
potential,  for  instance  on  80  volts — which  would  correspond 
to  3.9  amperes  on  curve  I — then  any  tendency  of  the  current  to 
increase — as  by  a  momentary  drop  of  the  arc  resistance — 
would  lower  the  required  arc  voltage,  and  so  increase  the  cur- 
rent, at  constant  supply  voltage,  hence  still  further  lower  the 
arc  voltage,  etc.,  and  a  short  circuit  would  result.  Vice  versa, 
a  momentary  decrease  of  arc  current,  by  requiring  more  volt- 
age than  is  available,  would  still  further  decrease  the  current, 
increase  the  required  voltage,  etc.,  and  the  arc  would  extin- 
guish. 

Therefore  only  such  apparatus  are  operative  on  constant 
potential,  in  which  an  increase  of  current  requires  an  increase 
of  voltage,  and  vice  versa;  and  so  limits  itself. 

While  therefore  arcs  can  be  operated  on  a  constant  cur- 
rent system,  to  run  arc  lamps  on  constant  potential,  some  cur- 
rent limiting  device  is  necessary  in  series  with  the  arc,  as  a 
resistance;  or,  in  an  alternating  current  circuit,  a  reactance. 

The  voltage  consumed  by  the  resistance  is  proportional  to 
the  current,  and  a  resistance  of  8  ohms  inserted  in  series  to  the 
arc  would  thus  consume  the  voltage  shown  in  straight  line  II  in 
Fig.  47.  The  voltage  consumed  by  the  arc  plus  .the  resistance 
then  is  given  by  the  curve  III,  derived  by  adding  I  and  II.  As 


218 


GENERAL  LECTURES 


seen,  below  3.35  amperes,  the  total  required  voltage  still 
decreases  with  increase  of  current,  and  the  arc  is  still  unstable ; 
that  is,  the  resistance  is  insufficient.  Above  this  current,  an 
increase  of  current  requires  an  increase  of  voltage  and  so 
limits  itself;  that  is,  the  arc  is  stable;  with  8  ohms  series  resist- 
ance, 3.35  amperes  therefore  is  the  limit  of  stability  of  the  arc; 
and  attempting  to  operate  it  at  lower  current,  as  for  instance 
at  2  amperes  and  106  volts  supply,  the  arc  either  goes  out,  or 


ARC  LIGHTING  219 

the  current  runs  up  to  5.5  amperes,  where  the  arc  becomes 
stable  on  106  volts  supply. 

With  a  higher  series  resistance,  the  arc  remains  stable  to 
lower  currents,  and  vice  versa.  It  follows  herefrom,  that  for 
the  operation  of  an  arc  lamp  on  constant  potential,  a  higher 
voltage  is  required  than  that  consumed  by  the  arc  proper. 

At  every  value  of  series  resistance  therefore  a  point  a  in 
Fig.  47,  is  reached,  at  which  for  decreasing  current  the  arc 
becomes  unstable;  and  all  these  points,  for  different  resistance 
values,  give  a  curve  IV,  which  is  called  the  "stability  curve" 
of  the  arc  curve  I. 

The  supply  voltage  required  to  operate  the  arc  represented 
by  curve  I  must  therefore  be  higher  (than  that  given  by  the 
stability  curve  IV.  For  instance,  at  4  amperes,  the  arc  cannot 
be  operated  at  less  than  104  volts  supply.  At  104  volts  supply 
the  limit  of  stability  is  reached ;  that  is,  a  change  of  current  does 
not  require  a  change  of  voltage,  but  the  arc  voltage  decreases 
as  much  as  the  resistance  voltage  increases  and  the  current 
thus  drifts;  and  for  supply  voltages  higher  than  104,  the 
arc  is  stable,  the  more  so,  the  higher  the  supply  voltage 
is  above  104.  The  difference  in  voltage  between  the  supply 
voltage  and  the  arc  voltage  thus  is  consumed  by  the  "steadying 
resistance"  of  the  arc. 

High  reactance  in  series  with  the  direct  current  arc 
retards  the  current  fluctuations  and  so  reduces  them;  so  .that 
with  reactance  in  series  to  the  direct  current  arc,  the  arc  can  be 
operated  by  a  supply  voltage  closer  to  the  stability  curve  IV 
than  without  reactance ;  reactance  therefore  is  very  essential  in 
the  steadying  resistance  of  a  direct  current  arc.  Obviously, 
no  series  reactance  can  enable  operation  of  the  arc  I  on  a 
supply  voltage  below  that  given  by  the  stability  curve  IV. 


220  GENERAL  LECTURES 

The  arc  characteristic  I  is  far  steeper  for  low  currents  than 
for  high  currents,  and  is  ithe  steeper  the  greater  the  arc  length. 
Low  current  arcs  and  long  arcs  therefore  require,  that  on  a 
constant  potential  supply,  a  greater  part  of  the  supply  voltage  is 
consumed  by  the  steadying  resistance  (or  steadying  reactance 
with  alternating  arcs)  than  high  current  arcs,  or  short  arcs; 
and  are  therefore  less  economical  on  constant  potential  supply. 

Constant  potential  arc  lamps  are  necessarily  less 
efficient  than  constant  current  arc  lamps,  due  to  the  power  con- 
sumed in  the  steadying  resistance.  A  large  part  of  this  power 
is  saved  in  alternating  constant  potential  arc  lamps,  by  using 
reactance  instead  of  resistance,  but  the  power  factor  is  there- 
fore greatly  lowered ;  that  is,  the  constant  potential  alternating 
arc  lamp  rarely  has  a  power  factor  of  over  70%. 

Where  therefore  high  potential  constant  current  circuits 
are  permissible,  as  for  outdoor  or  street  lighting,  arc  lamps 
are  usually  operated  on  a  constant  current  circuit,  with  series 
connection  of  from  50  to  100  lamps  on  one  circuit.  With  the 
exception  of  a  few  of  the  larger  cities,  all  the  street  lighting 
by  arc  lamps  in  this  country  is  done  by  constant  current 
systems,  either  direct  current  or  alternating  current. 

For  direct  current  constant  current  supply,  separate  arc 
light  machines  have  been  built,  and  are  still  largely  used.  In 
these  machines,  inherent  regulation  for  constant  current  is 
produced  by  using  a  very  high  armature  reaction  and  relatively 
weak  field  excitation;  that  is,  the  armature  ampere  turns  are 
nearly  equal  and  opposite  to  the  field  ampere  turns,  and  thus 
both  very  large  compared  with  the  difference,  the  resultant 
ampere  turns,  which  produce  the  magnetic  field.  A  moderate 
increase  of  current  and  consequent  increase  of  armature  ampere 
turns  therefore  greatly  reduces  the  resultant  ampere  turns  and 


ARC  LIGHTING  221 

so  the  field  magnetism  and  the  voltage,  tthat  is,  the  machine 
tends  to  regulate  for  constant  current.  Perfect  constant  current 
regulation  then  is  secured  by  some  governing  device,  as  an  auto- 
matic regulator  varying  a  resistance  shunted  across  the  series 
field.  It  must,  however,  be  understood  .that  the  "regulator" 
of  the  arc  machine  does  not  give  a  constant  current  regula- 
tion, but  the  armature  reaction  of  the  machine  does  this,  and 
the  regulator  merely  makes  it  perfect;  but  even  with  the  regu- 
lator disconnected,  arc  machines  give  fairly  close  constant  cur- 
rent regulation. 

As  the  voltages  produced  by  arc  machines  are  very  high— 
4,000  to  10,000 — commutation  of  the  current,  with  the 
ordinary  commutator,  which  is  limited  to  a  maximum  of  40 
to  50  volts  per  segment — -is  not  well  suited,  but  rectification 
is  used.  The  Brush  arc  machine  therefore  is  a  quarter-phase 
alternator  with  rectifying  commutator.  That  is,  the  commuta- 
tor shifts  the  connection  over  from  the  phase  of  falling  e.  m.  f. 
to  that  of  rising  e.  m.  f.,  and  thereby  is  able  to  control  as  high 
as  3,000  volts  per  commutator  ring. 

With  the  development  of  the  mercury  arc  rectifier,  which 
converts  constant  alternating  current  into  constant  direct  cur- 
rent, arc  machines  are  rapidly  going  out  of  use.  The  arc 
machine  necessarily  must  be  a  small  unit,  since  100  to  150 
lamps  in  series  give  already  as  high  a  voltage  as  is  safe  to  use  in 
arc  circuits,  but  do  not  yet  represent  much  power;  and  when 
supplying  thousands  of  arc  lamps  a  large  number  of  small 
machine  units  are  required,  which  are  uneconomical  in  space, 
in  attendance  and  in  efficiency.  The  mercury  arc  rectifier  in 
combination  with  the  stationary  constant  current  transformer 
enables  us  to  derive  the  power  from  the  alternating  current  con- 
stant potential  supply  system. 


222 


GENERAL  LECTURES 


Constant  alternating  current  is  derived  by  a  constant  cur- 
rent transformer  or  constant  current  reactance.  Diagram- 
matically,  the  constant  current  transformer  is  shown  in  Fig. 
48.  The  primary  coil  P  and  the  secondary  coil  S  are  movable 


with  regard  to  each  other  (which  of  the  two  coils  is  movable, 
is  immaterial,  or  rather,  is  determined  by  consideration  of 
design).  Fig.  48  shows  the  coil  S  suspended  and  its  weight 
partially  balanced  by  counter-weight  W. 

With  the  secondary  coil  S  close  to  the  coil  P,  that  is,  in 
the  lowest  position,  most  of  the  magnetism  produced  by  the 
primary  coil  P  passes'  through  the  secondary  coil  S,  and  the 
secondary  voltage  therefore  is  a  maximum.  The  further  the 
secondary  coil  moves  away  from  the  primary  coil,  the  more  of 
the  magnetism  passes  between  the  coils,  the  less  through  ,the 
secondary  coil,  and  the  lower  therefore  is  the  secondary  voltage, 


ARC  LIGHTING  223 

which  becomes  a  minimum  (or  zero,  if  so  desired),  with  the 
secondary  coil  at  a  maximum  distance  from  the  primary,  that 
is,  in  the  top  position. 

Primary  current  and  secondary  current  are  proportional 
and  in  opposition  to  each  other,  and  repel  each  other,  and  the 
repulsion  is  proportional  to  the  product  of  the  two  currents; 
that  is,  proportional  to  the  square  of  the  secondary  current. 
The  weight  of  the  secondary  coil  is  balanced  by  the  counter- 
weight W  and  -the  repulsion  from  the  primary  coil,  at  normal 
secondary  current.  Any  increase  of  secondary  current  by  a 
decrease  of  load,  increases  the  repulsion,  in  this  way  pushing 
the  secondary  coil  further  away  from  the  primary  and  thereby 
reducing  the  secondary  voltage  and  thus  the  current;  and  vice 
versa,  a  decrease  of  secondary  current,  by  an  increase  of 
load,  reduces  the  repulsion  and  so  causes  the  secondary  coil  tc 
come  nearer  to  the  primary,  that  is,  increases  its  voltage  and  so 
restores  the  current.  Such  an  arrangement  regulates  for  con- 
stant current  between  the  voltage  limits  given  by  the  two  ex- 
treme positions  of  the  movable  coil.  These  usually  are  chosen 
from  some  margin  above  full  load,  down  to  about  one-third 
load. 

The  constant  current  reactance  operates  on  the  same 
principle :  the  two  coils  P  and  S  are  connected  in  series  with 
each  other  into  the  arc  circuit  supplied  from  the  constant 
potential  source,  and  by  separating  or  coming  together, 
vary  in  reactance  with  the  load,  and  thereby  maintain  constant 
current. 

While  the  alternating  current  arc  lamp  is  less  efficient, 
that  is,  gives  less  light  for  the  same  power,  than  the  direct  cur- 
rent arc  lamp,  the  disadvantages  of  the  use  of  numerous  arc 
machines  have  led  -to  the  extended  adoption  of  alternating  cur- 
rent series  arc  lighting  before  the  development  of  the  mercury 


224  GENERAL  LECTURES 

arc  rectifier,  which  enabled  the  operation  of  direct  current  arc 
circuits  from  constant  current  transformers. 

While  incandescent  lamps  give  the  same  efficiency  for  all 
sizes  except  such  small  sizes  where  mechanical  difficulties 
appear  in  the  filament  production,  the  efficiency  of  the  arc 
decreases  greatly  with  decrease  of  current;  that  is,  the  arc  is  at 
the  greatest  efficiency  only  for  large  units  of  light,  but  rather 
inefficient  and  not  so  well  suited  for  small  units  of  light. 

Even  in  large  units,  the  efficiency  of  light  production  of 
the  direct  current  carbon  arc  lamp  is  not  superior  to  that  of 
the  tungsten  incandescent  lamp ;  that  of  the  alternating  current 
carbon  arc  lamp  is  inferior  to  the  tungsten  lamp ;  and  the  carbon 
arc  lamp  thus  finds  its  field  mainly  where  large  units  of  light  are 
required,  especially  as  long  as  the  cost  of  renewal  of  the 
metal  filament  lamps  is  still  very  great.  Entirely  different, 
however,  are  ithe  conditions  developed  in  the  last  years,  with  the 
luminous  arcs,  as  the  flame  carbon  arc,  the  mercury  lamp,  and 
the  magnetite  and  titanium  carbide  arc.  In  these,  efficiencies 
of  light  production  have  been  reached  which  no  incandescent 
lamp  can  hope  .to  approach. 

In  the  carbon  arc,  practically  all  the  light  comes  from  the 
incandescent  tips  of  the  carbons,  very  little  from  the  arc  flame. 
Then  by  using  materials,  which  in  the  arc  flame  give  an  intense- 
ly luminous  spectrum,  the  efficiency  of  die  arc  lamp  has  been 
vastly  improved.  j 

So  far  only  three  materials  have  been  found,  which  in 
luminous  arcs  give  efficiences  vastly  superior  to  incandescence : 
mercury,  calcium  (lime),  and  titanium.  All  three  even  in 
moderate  sized  units,  give  efficiencies  of  one-half  watt  or  better 
per  candle  power. 

The  mercury  arc  has  the  advantage  of  perfect  steadiness, 
a  long  life — requiring  no  attention  for  thousands  of  hours — 


ARC  LIGHTING  225 

and  high  efficiency  over  a  fairly  wide  range  of  candle  powers ; 
but  it  is  seriously  handicapped  for  many  purposes  by  its  bluish- 
green  color. 

In  the  flame  carbon  lamp  carbons  impregnated  with  cal- 
cium compounds,  usually  calcium  fluoride  (fluorspar)  are 
used,  and  the  arc  then  has  an  orange-yellow  color.  Other  com- 
pounds which  give  red  or  white  color  to  the  arc  are  so  much 
inferior  in  efficiency  that  they  are  used  only  to  a  very  limited 
extent.  The  compounds,  after  coloring  the  arc  and  giving  it 
efficiency,  escape  as  smoke;  the  arc  therefore  must  be  an  open 
arc.  This,  however,  means  short  life  of  the  carbons  and  fre- 
quent trimming. 

The  open  arc  lamp,  which  was  used  formerly,  has,  how- 
ever, been  almost  entirely  superseded  by  the  enclosed  carbon 
arc,  in  spite  of  the  somewhat  lower  efficiency  of  the  latter ;  and 
the  inconvenience  of  daily  attendance  required  by  an  open  arc, 
and  the  large  consumption  of  carbons,  makes  a  return  to  this 
type  improbable.  For  this  reason  the  flame  carbon  lamp  has 
not  proven  suitable  for  general  outdoor  illumination,  as 
street  lighting,  where  the  cost  of  carbons  and  trimming 
would  usually  far  more  than  offset  the  gain  in  efficiency. 
Flame  carbon  lamps,  however,  have  found  a  field  for  decora- 
tive lighting,  for  advertising  purposes,  etc.,  for  which  the  glare 
of  light  and  its  color  makes  them  very  suitable.  They  are 
generally  used  on  constant  potential  circuits  with  two  or  three 
lamps  in  series. 

To  eliminate  the  objections  of  short  life  and  consequent 
frequent  trimming  and  high  cost  of  carbons,  and  thereby  make 
the  luminous  arc  able  to  enter  the  field  of  general  outdoor  il- 
lumination, carbon  had  to  be  eliminated  altogether  as  electrode 
material,  and  its  place  was  taken  by  magnetite,  while  titanium 
compounds  give  the  high  efficiency.  This  lead  to  the  long 


226  GENERAL  LECTURES 

burning  luminous  arc  of  the  white  color  of  the  titanium-iron 
spectrum  as  represented  by  the  magnetite  arc,  the  metallic 
oxide  arc,  and  other  types  still  in  development. 

In  all  these  long  burning  luminous  arcs,  some  efficiency 
had  to  be  sacrificed  in  developing  sufficiently  small  units  for 
general  illumination.  While  the  substitution  of  the  flame  car- 
bon in  the  open  arc  has  quadrupled  the  light  at  the  same  power 
consumption,  and  the  substitution  of  the  magnetite  electrode 
for  carbon  at  the  same  power  consumption  would  in  the  same 
manner  increase  the  light,  for  street  illumination  the  main 
problem  was,  -to  decrease  the  power  consumption  rather  than 
increase  the  amount  of  light  given ;  and  so  in  the  long  burning 
luminous  arcs,  which  are  now  beginning  to  replace  the  carbon 
arcs  of  old,  the  power  consumption  has  been  reduced  by  from 
30  to  60%  with  a  sufficient  increase  of  light  to  be  marked. 

In  the  arc  lamp,  the  current  is  carried  across  the  gap 
between  the  terminals  by  a  stream  of  vapor  of  the  electrodes ; 
thus  the  electrodes  consume  more  or  less  rapidly.  Some 
feeding  mechanism  is  therefore  required  to  move  the  electrodes 
towards  each  other  during  their  consumption.  This  arc  lamp 
mechanism  may  be  operated  by  the  current,  or  by  the  voltage, 
or  by  both;  this  gives  the  three  different  types  of  lamps:  the 
series  lamp,  the  shunt  lamp,  and  the  differential  lamp. 

In  the  series  lamp,  an  electromagnet  energized  by  the 
lamp  current,  and  balanced  against  a  weight  or  a  spring,  moves 
the  carbons  towards  each  other  when  by  their  burning  off,  the 
arc  lengthens  and  the  current  decreases.  Obviously,  this  lamp 
cannot  be  used  on  constant  current  circuits,  or  with  several 
lamps  in  series,  but  only  as  single  lamp  on  constant  potential 
circuits,  and  therefore  has  practically  disappeared. 

In  the  shunt  lamp,  the  controlling  magnet  is  shunted 
across  the  arc,  and  with  increasing  arc  length  and  consequent 


ARC  LIGHTING  227 

arc  voltage,  moves  the  electrodes  towards  each  other.  In  con- 
stant current  circuits,  this  lamp  tends  towards  hunting,  and 
therefore  requires  a  very  high  reactance  in  series;  it  thereby 
gives  a  lower  power  factor  in  alternating  current  circuits,  and 
has  therefore  been  superseded  by  the  differential  lamp.  It  has, 
however,  the  advantage  of  not  being  sensitive  to  changes  of 
current. 

In  the  differential  lamp,  an  electromagnet  in  series  with 
the  arc  opposes  an  electromagnet  in  shunt  to  the  arc,  and  the 
lamp  regulates  for  constant  arc  resistance.  It  is  the  lamp 
now  universally  used  in  constant  potential  and  constant  cur- 
rent systems,  is  most  stable  in  its  operation;  but  in  constant 
current  systems,  it  requires  that  the  current  be  constant  within 
close  limits :  if  the  current  is  low,  the  arc  is  too  short,  and  the 
lamp  gives  very  little  light,  and  if  the  current  is  high,  the  arc 
becomes  so  long  as  to  endanger  the  lamp. 

From  the  operating  mechanism  the  motion  is  usually 
transmitted  to  the  electrode  by  a  clutch,  which  releases  and  lets 
the  electrodes  slip  together. 

In  the  carbon  arc  lamp,  the  mechanism  is  "floating" ;  that 
is,  the  upper  carbon,  held  by  the  opposing  forces  of  shunt  and 
series  magnets,  moves  with  every  variation  of  the  arc  resist- 
ance, and  so  maintains  very  closely  constant  voltage  on  the  arc. 
In  the  long  burning  luminous  arc,  as  the  magnetite  lamp, 
which  the  light  comes  from  the  arc  flame,  and  thus  constant 
length  of  arc  flame  is  required  for  constant  light  production. 
The  floating  mechanism,  which  constantly  varies  the  arc  length 
with  the  variation  of  the  arc  resistance,  has  therefore  been 
superseded  by  a  mechanism  which  sets  the  arc  at  fixed  length, 
and  leaves  it  there  until  with  the  consumption  of  the  electrodes 
the  arc  has  sufficiently  lengthened  to  cause  the  shunt  coil  to 


228  GENERAL  LECTURES 

operate  and  to  reset  the  arc  length.  Thus  in  some  respects, 
these  lamps  are  shunt  lamps. 

During  the  early  days  of  the  open  carbon  arc  lamp,  9.6, 
6.6  and  4  amperes  were  the  currents  used  in  direct  current 
arc  circuits,  with  about  40  volts  per  lamp.  The  4  ampere  arc 
very  soon  disappeared,  as  giving  practically  no  light. 

In  the  enclosed  arc  lamp,  the  carbons  are  surrounded  by 
a  nearly  air  tight  globe,  which  restricts  the  admission  of  air 
and  thus  the  combustion  of  the  carbon,  and  so  increases  ithe  life 
of  the  carbons  from  8  or  10  hours  to  70  to  120  hours.  In  these 
lamps,  lower  currents  and  higher  arc  voltages,  that  is,  longer 
arcs,  are  used :  in  direct  current  circuits,  6.6  amperes  and  5 
amperes,  with  70  to  75  volts  per  lamp;  in  alternating  current 
circuits,  7.5  and  6.6  amperes  are  used  with  the  same  arc 
voltage. 

In  the  direct  current  magnetite  arc  lamp,  4  amperes  and  75 
to  80  volts  per  lamp  are  used ;  in  the  alternating  current  titan- 
ium carbide  arc  lamp,  only  2.5  amperes  and  80  to  85  volts  per 
lamp  are  used. 


APPENDIX  I 


LIGHT  AND  ILLUMINATION 

Paper  read  before  the  Illuminating  Engineering 
Society,  December  14,  1906. 

REVISED  TO  DATE. 

I. 

OMPARED  with  other  branches  of  engineering,  as  the 
transformation  of  electrical  power  into  mechanical 
power  in  the  electric  motor,  or  the  transformation  of 
chemical  into  mechanical  energy  in  the  steam  engine,  we  are 
at  the  disadvantage  when  dealing  with  light  and  illumination, 
that  we  have  not  to  do  any  more  strictly  with  a  problem  of 
physics,  but  that  we  are  on  .the  borderland  between  applied 
physics  that  is  engineering,  and  physiology.  Light  is  not  a 
physical  quantity,  but  it  is  the  physiological  effect  exerted  upon 
the  human  eye  by  certain  radiations. 

There  are  different  forms  of  energy,  all  convertible  into 
each  other,  as  magnetic  energy,  electric  energy,  heat  energy, 
mechanical  momentum,  radiating  energy,  etc.  The  latter,  radi- 
ating energy,  is  a  vibratory  motion  of  a  hypothetical  medium, 
the  ether,  which  vibration  is  transmitted  or  propagated  at  a 
velocity  of  about  188,000  miles  per  second;  and  it  is  a 
transverse  vibration,  differing  from  the  vibratory  energy  of 
sound  in  this  respect,  that  the  sound  waves  are  longitudinal, 
that  is,  the  vibration  is  in  the  direction  of  the  beam,  while  the 
vibration  of  radiation  is  transverse. 

Radiating  energy  can  be  derived  from  other  forms  of 
energy,  for  instance,  from  heat  energy  by  raising  a  body  to  a 


23o  GENERAL  LECTURES 

high  temperature.  Then  the  heat  energy  is  converted  into  radi- 
ation and  issues  from  the  heated  body,  as  for  instance  an  incan- 
descent lamp  filament,  as  a  mass  of  radiations  of  different  wave 
lengths,  that  is,  different  frequencies.  All  kinds  of  frequencies 
appear :  from  very  low  frequencies,  that  is  only  a  few  millions 
of  millions  of  cycles  per  second,  up  to  many  times  higher 
frequencies.  We  can  get,  if  we  desire,  still  very  much  lower  fre- 
quencies, as  electromagnetic  waves,  such  as  the  radiation  sent 
out  by  an  oscillating  current  or  an  alternating  current ;  but  the 
radiations  which  we  get  from  heated  bodies  are  all  of  extremely 
high  frequency,  compared  with  the  customary  frequencies  of 
electric  currents.  At  the  same  time  .they  cover  a  very  wide 
range  of  frequencies,  many  octaves,  and  from  all  this  mass  of 
radiations,  from  all  the  frequencies  of  radiating  energy,  some- 
what less  than  one  octave  can  be  perceived  by  the  human  eye  as 
light. 

Light,  therefore  is  the  physiological  effect  exerted  upon 
the  human  eye  by  a  certain  narrow  range  of  frequencies  of 
radiation.  Frequencies  lower  than  those  visible  to  the  eye, 
and  frequencies  higher  than  those  visible  to  the  eye,  are  again 
invisible. 

We  frequently  speak  of  those  frequencies  which  are 
lower  than  the  visible  ones,  as  radiating  heat,  and  of  those 
frequencies  higher  than  the  visible  ones  as  chemical  rays. 
This,  however,  is  misleading,  and  there  is  no  distinction  in 
character  between  radiations  of  different  frequency.  There 
are  no  heat  rays  differing  from  light  rays  of  chemical  rays. 
Any  form  of  energy  when  destroyed  gives  rise  to  an  exactly 
equivalent  amount  of  some  other  form  of  energy.  If  there- 
fore we  destroy  radiating  energy  by  intercepting  the  beam  of 
radiation  by  interposing  an  opaque  body  in  its  path,  then  the 
energy  of  radiation  is  converted  into  some  other  form  of 


LIGHT  AND  ILLUMINATION 

energy,  usually  into  heat.  That  means  that  any  radiation 
when  absorbed  produces  heat  and  the  amount  of  heat  pro- 
duced merely  represents  the  amount  of  energy  which  was  con- 
tained in  the  radiation.  If  the  radiation  contains  a  very 
large  amount  of  energy,  the  heat  evolved  by  intercepting 
it  may  be  sufficient  to  be  felt  by  putting  your  hand  in  the  beam. 
If  the  amount  of  energy  is  less,  it  may  not  be  possible  to  feel 
it,  though  with  a  sensitive  instrument,  as  a  bolometer,  we  may 
still  be  able  to  measure  the  heat.  All  radiations  therefore  are 
convertible  into  heat:  the  visible  light  waves  as  well  as  the 
invisible  ultraviolet  rays,  and  the — usually  more  powerful — 
long  ultrared  waves ;  but  none  of  the  radiations  can  be  called 
heat,  no  more  than  the  mechanical  momentum  of  a  flywheel  is 
heat,  because  when  destroyed,  it  produces  heat. 

If  we  consider  the  infinite  range  of  radiation  issuing  from 
heated  bodies,  we  find  that  those  rays  which  are  of  lower  fre- 
quency than  the  visible  rays  will  be  felt  as  heat,  because  they 
contain  a  very  large  amount  of  energy.  The  rays  which  are 
visible  represent  very  little  energy — and  therefore  they 
do  not  give  as  much  heat.  For  instance,  in  the  case  of  a  hot 
steam  boiler,  although  we  get  no  light,  we  can  feel  the  radia- 
tion from  it  by  the  heat  which  it  produces  when  intercepted  by 
our  hand  held  near  it.  We  do  not  feel  the  radiation  as  heat 
which  issues  from  the  green  light  of  the  mercury  lamp,  merely 
because  the  energy  of  radiation  in  the  latter  is  less  than  the 
amount  of  energy  in  the  radiation  .f rom  a  hot  steam  boiler ;  but 
while  it  is  less  in  the  former  case,  it  happens  to  be  of  that  fre- 
quency which  affects  the  eye  and  is  visible. 

As  a  consequence,  when  we  speak  of  cold  light,  this  does 
not  mean  that  it  is  different  from  hot  light — from  the  light, 
for  instance,  given  by  a  hot  coal  fire,  where  we  feel  the  radia- 
tion as  heat;  it  merely  means  that  what  is  usually  called  cold 


232  GENERAL  LECTURES 

light  (as  the  light  of  the  firefly  is  supposed  to  be)  is  radia- 
tion containing  to  a  very  large  extent  rays  of  the  visible 
frequencies  and  not  much  energy  outside  of  the  visible  range ; 
i.  e.,  containing  very  little  total  energy,  so  that  the  energy 
when  destroyed,  that  is,  converted  into  heat,  cannot  be  felt 
easily,  but  requires  more  delicate  methods  of  determination; 
while  a  very  inefficient  light,  as  a  coal  fire  for  instance,  which 
gives  most  of  its  energy  as  invisible  radiation  of  low  frequency, 
very  little  as  visible  radiation,  can  be  felt  by  the  heat  pro- 
duced by  the  interception  of  the  rays,  mainly  the  energetic  low 
frequency  rays.  As  stated,  then,  there  is  no  essential  differ- 
ence between  so-called  heat  waves  and  light  waves,  but  any 
radiation  can  be  converted  into  other  forms  of  energy,  .the  so- 
called  chemical  rays  of  ultraviolet  light,  the  X-ray,  as  well 
as  the  ultrared  and  the  visible  rays,  and  when  converted  into 
heat  can  be  noticed  as  such.  Now  it  just  happens  that  most 
of  our  means  of  producing  radiating  energy  give  high  intensi- 
ties of  radiation  only  for  very  low  frequencies,  invisible  ultra- 
red  rays,  but  we  are  not  able  to  produce  anywhere  near  the 
same  intensities  of  radiation  for  higher  frequencies. 

So  also,  when  we  speak  of  ultraviolet,  or  short,  high 
frequency  waves,  as  chemical  waves,  that  does  not  mean  that 
they  have  a  distinctive  character  in  producing  chemical 
action — any  form  of  energy,  naturally,  can  be  converted  if 
we  know  how,  into  chemical  energy,  the  long  ultrared  waves 
just  as  well  as  the  short  ultraviolet  waves.  It  just  happens 
that  those  chemical  compounds  which  are  easily  split  up  by 
radiating  energy,  are  silver  salts  or  salts  of  gold  and  platinum ; 
they  are  especially  affected  by  the  ultraviolet  and  violet  rays. 
We  observe,  then,  the  chemical  action  of  these  rays,  but  do  not 
observe  so  well  the  chemical  action  of  other  rays.  There  may, 
however,  be  some  feature  in  the  constitution  of  matter,  which 


LIGHT  AND  ILLUMINATION  233 

accounts  for  the  high  chemical  action  of  the  ultraviolet  and 
violet  rays. 

It  is  obvious  that  if  we  intercept  and  destroy  radiations 
and  so  convert  their  energy  into  other  forms  of  energy,  if  .the 
energy  is  only  great  enough,  we  get  a  high  temperature,  and 
thus  a  high  chemical  action,  merely  by  the  effect  of  temperature. 
But  we  may  also  get  a  chemical  effect  by  what  probably  is 
some  kind  of  a  resonance  phenomenon.  The  particles  of  a  body, 
atoms  or  molecules,  must  have  some  rate  of  vibration  of  their 
own.  If,  then,  a  ray  of  radiation  impinges  upon  them  which 
is  of  a  frequency  of  the  same  magnitude  as  the  inherent  rate  of 
vibration  of  -the  atom,  by  resonance  this  vibration  of  the  atom 
must  rapidly  increase  in  intensity  until  the  atom  breaks  away 
from  the  others,  or  the  molecule  breaks  up,  that  is,  the  chemical 
combination  is  split  up. 

The  inherent  frequency  of  oscillation  of  the  atom  seems 
to  be  of  about  of  the  same  magnitude  as  the  visible  radia- 
tion, or  rather  of  a  little  higher  frequency;  that  is,  if  the 
atoms  are  left  to  vibrate  freely  as  under  the  influence  of 
an  electric  current  in  the  arc,  then  we  get  radiations  of  the 
frequency  inherent  to  the  atom.  The  general  tendency  then  is 
toward  the  violet  or  short  wave  end  of  the  spectrum.  If  we 
assume  -that  the  mass  of  the  silver  atom  is  such  as  to  give 
a  rate  of  vibration  in  the  range  of  the  violet  and  ultraviolet, 
it  is  easy  to  understand  that  radiation  of  this  frequency  splits 
up  the  silver  salt  by  increasing  -the  vibration  of  the  atom  by 
resonance,  and  that  shorter  or  longer  waves  have  no  effect,  or 
much  less  effect.  So  it  may  be  a  mere  incident  that  those 
chemical  compounds  on  which  we  observe  the  chemical  action 
of  radiation  just  happen  to  be  sensitive  to  the  violet  end  of  the 
spectrum.  It  is  indeed  a  fact  that  other  chemical  changes 
brought  about  by  radiating  energy,  as  the  formation  of  ozone 


234  GENERAL  LECTURES 

from  oxygen,  that  is,  the  splitting  up  of  the  oxygen  molecule 
and  reforming  of  the  ozone  molecule  from  the  atoms,  do  not 
take  place  in  the  violet  or  ultraviolet,  but  requires  frequencies 
very  much  higher,  about  the  highest  frequencies  which  the 
mercury  arc  at  low  temperature  gives.  Possibly,  since  the 
oxygen  atom  is  so  much  lighter  than  the  silver  atom,  its  fre- 
quency of  vibration  is  much  higher,  which  means  that 
resonance  effects  and  destruction  of  the  molecules  take  place 
only  with  a  much  shorter  wave  length  of  radiation,  or  much 
higher  frequency.  ( 

Vice  versa,  it  seems  that  these  frequencies  which  are 
chemically  active  in  organic  life,  which  give  the  energy 
absorbed  from  radiation  by  plants,  and  so  the  chemical  activity 
utilized  in  building  up  the  growth  of  vegetation,  are  not  at 
the  violet,  but  at  the  red  end  of  the  spectrum.  It  appears 
that  the  red  and  ultrared  rays  produce  growth  of  plants 
and  the  chemical  activity  which  we  call  life,  while  the  violet 
and  ultraviolet  rays  kill.  Even  ithis  we  can  well  under- 
stand if  we  consider  the  chemical  activity  as  a  resonance 
phenomenon,  because  the  metabolism  of  protoplasm  which  we 
call  life,  is  based  on  the  existence  of  unstable  structures  of 
carbon  compounds.  We  have  here  not  atoms  combining  with 
each  other,  but  groups,  chain  and  ring  formations,  which 
are  of  larger  mass  and  therefore  have  a  lower  rate  of  vibration 
and  so  should  be  expected  to  respond  .to  lower  frequencies  or 
to  red  light,  as  indeed  seems  to  be  the  case.  The  violet  and 
ultraviolet  light  does  not  split  up  the  organic  matter  into 
groups,  which  recombine  to  form  complex  bodies,  and  so 
represent  the  changes  called  life;  but  due  to  its  higher  fre- 
quency, resonance  occurs  with  the  atoms,  that  is,  the  organic 
compound  splits  up  into  atoms  and  so  disintegrates,  which 
means  death. 


LIGHT  AND  ILLUMINATION  235 

So  it  can  be  understood  that  the  chemical  activity  of 
different  radiations  may  be  different;  the  chemical  activity  of 
long  rays  gives  life  to  the  vegetation  and  the  short  waves, 
death;  one  splits  up  into  carbon  groups  and  .the  other  carries 
destruction  down  to  the  atom. 

The  popular  distinction  between  heat  waves,  chemical 
waves  and  light  waves,  therefore  is  not  a  physical  distinction, 
but  all  are  radiating  energy  of  the  same  character,  differing 
merely  in  wave  length,  and  the  visible  range  is  somewhat  less 
than  one  octave,  rather  at  the  upper  end,  at  the  higher  fre- 
quencies, which  are  difficult  to  produce.  This  makes  the  prob- 
lem of  investigating  and  dealing  with  light  difficult  for  the  engi- 
neer, because  it  is  not  any  more  a  physical  quantity  which  can  be 
measured  accurately,  as  in  .the  case  of  power  or  velocity,  but 
it  is  a  physiological  effect.  We  can,  indeed,  measure  very 
accurately  the  total  energy  of  radiation  from  a  heated  body, 
but  the  total  energy  of  radiation  is  not  light :  only  a  very  small 
part  of  it  is  visible.  We  can  go  further  and  split  up  the  total 
radiation  issuing  from  a  hot  body,  as  the  incandescent  lamp  fila- 
ment, into  its  different  wave  lengths  and  different  frequencies ; 
as  for  instance,  we  can  resolve  the  (total  radiation  into  the 
spectrum  by  using  a  prism  to  separate  the  different  frequencies, 
and  then  collect  the  total  of  the  radiation  within  the  visible 
range,  by  a  lens  or  other  means,  and  measure  all  the  energy  of 
the  visible  radiation.  Or,  still  simpler,  although  approximate, 
we  may  interpose  in  the  beam  of  radiation  some  medium  which 
absorbs  the  invisible  long  rays  and  invisible  short  rays,  and 
which  transmits,  all  or  rather  most,  of  the  visible  rays,  as  for 
instance  glass  and  water.  In  this  manner  one  could  easily 
measure  .the  energy  of  the  visible  radiation,  and  compare  the 
energy  of  the  visible  radiation  with  the  total  energy  producing 
this  radiation.  That  would  give  a  physical  measure  of  the 


236  GENERAL  LECTURES 

efficiency  of  producing  visible  radiation  but  it  would  not  be  a 
measure  of  the  efficiency  of  producing  light,  since  unfortunately 
the  different  wave  lengths  of  visible  radiation  are  very  differ- 
ent in  their  physiological  effect.  The  same  amount  of  energy  as 
visible  radiation,  giving  the  effect  of  green  light,  represents 
an  entirely  different  amount  of  light,  a  many  times  greater 
physiological  effect  than  the  same  amount  of  energy  as  red 
rays,  that  is,  rays  of  the  wave  lengths  which  give  the  impres- 
sion of  red  light. 

That  means,  the  physiological  effect  or  light-equivalent 
of  mechanical  energy  within  the  visible  range — is  a  function  of 
the  wave  length  and  varies  with  the  wave  length,  that  is,  with 
the  color.  That  really  is  obvious,  if  you  think  of  it:  if  you 
follow  the  range  of  frequency  from  a  low  frequency  to  a  high 
frequency,  you  see  that  energy  radiating  at  low  frequency 
represents  no  light  whatever,  has  no  physiological  equivalent, 
is  invisible.  When  you  come  into  the  visible  range  it  has  a 
physiological  effect.  Therefore,  when  you  pass  from  the  invis- 
ible into  the  visible  range,  the  physiological  equivalent  must 
pass  from  zero  into  a  finite  value  and  must  necessarily  pass 
continuously,  tthat  is,  at  the  extreme  end  of  the  visible  range; 
the  light  equivalent  of  energy  must  be  extremely  low,  and  the 
further  you  go  into  the  visible  range,  the  greater  it  is,  reaching 
the  maximum  in  the  middle  of  the  visible  range,  in  the  green 
and  yellow,  and  decreasing  again  down  to  zero  at  the  violet  end 
of  the  visible  rays ;  beyond  that,  at  still  higher  frequencies,  the 
physiological  equivalent  of  energy  is  zero  again ;  or,  vice  versa, 
if  we  consider  the  mechanical  equivalent  of  light,  it  is  a  mini- 
mum in  the  middle  of  the  visible  range,  where  one  candle 
power  of  light  represents  the  lowest  amount  of  energy,  and 
increases  toward  the  ends  of  the  spectrum  of  the  visible  range, 
to  infinity  at  the  ends  of  the  visible  range. 


LIGHT  AND  ILLUMINATION  237 

Now,  that  means,  in  plain  language,  that  the  efficiency  of 
light  production  is  a  function  of  the  wave  length,  that  is  of 
the  color,  and  that  it  is  at  its  maximum  in  the  middle  of  the 
spectrum,  where  the  same  amount  of  power,  measured  in 
watts,  gives  the  largest  amount  of  light  measured  in  candle 
power.  So  the  efficiency  of  light  production  is  a  function  of 
the  wave  length. 

Unfortunately,  the  physiological  equivalent  of  power,  or 
the  physiological  effect  of  light  varies  not  only  with  the 
wave  length,  but  also  with  the  absolute  intensity.  Suppose 
we  undertake  to  compare  red,  yellow  and  green  lights,  or  any 
lights  of  different  colors.  First  we  meet  great  difficulties  in 
comparing  them.  We  want  one  candle  power  in  light,  as  red, 
yellow,  or  green.  You  cannot  compare  different  colors  of 
light  directly,  since  there  is  no  physical  measure  of  light. 
Lights  are  compared  with  a  standard  lamp,  which  has  a  cer- 
tain color,  yellowish  white.  A  light  of  the  same  color  we  can 
compare  exactly;  if  the  color  is  not  much  different,  we  still 
get  an  approximate  comparison;  but  with  widely  different 
colors,  we  obviously  can  not  get  even  an  approximate  com- 
parison, can  not  say  when  the  two  sides  of  the  photometer 
screen,  one  illuminated  by  green  light,  the  other  by  red  light, 
are  equal  in  intensity.  There  is  thus  no  direct  comparison  of 
differently  colored  lights.  You  have  then  to  go  one  step  farther 
and  consider  that  light  is  used  for  illumination,  is  used  to  see 
by,  and  this  gives  you  a  fair  comparison :  you  observe  at  what 
distance  from  the  two  lights,  red  and  green,  you  can  read  with 
the  same  convenience,  read  the  same  kind  of  print,  or  to  meas- 
ure more  exactly,  get  the  maximum  distance  at  which  you 
can  just  read  a  certain  size  of  print,  by  either  light.  At  that 
distance  the  two  illuminations  are  the  same,  and  the  two  lights 
so  have  an  intensity  inverse  proportional  to  the  square  of  .these 


238  GENERAL  LECTURES 

distances.  In  this  manner  lights  of  different  color  can  be  com- 
pared. 

Necessarily,  the  comparison  has  not  the  accuracy  of 
photometrical  comparison.  This  cannot  be  expected,  since 
you  do  not  compare  physical  quantities,  but  only  physiological 
effects  on  the  eye,  and  different  observers  may  have  different 
personal  contants.  The  eye  of  the  one  may  be  more  sensitive 
to  green,  and  the  eye  of  the  other  may  be  more  sensitive  to 
red,  and  therefore  the  comparison  may  be  different.  However, 
these  individual  differences  are  not  great,  and  different 
observers,  even  with  widely  different  colors  of  light,  do  not 
give  results  differing  much  from  each  other,  so  that  a  com- 
parison of  intensities  of  differently  colored  lights,  and  thereby 
a  measurement  of  the  intensity  of  differently  colored  lights  in 
candle  power  is  feasible,  by  some  such  method,  that  is,  of 
observing  the  illumination  produced  by  the  different  lights. 

You  find,  however,  if  you  have  a  green  light  and  a  red 
light,  which  at  a  certain  distance  appear  equally  brilliant  to 
the  eye,  then  when  you  get  nearer  to  the  two  lights  the  orange 
red  light  appears  much  brighter  than  the  green,  and  when  you 
go  further  away  the  green  light  appears  brighter,  and  at  still 
greater  distances  you  still  see  the  green  light  fairly  brightly, 
while  the  red  light  is  almost  invisible.  That  is,  the  relative 
physiological  effect  of  different  wave  lengths  varies,  not  only 
with  the  wave  lengths,  but  also  with  the  absolute  intensity  of 
illumination,  and  while  throughout  the  whole  range  the  sensi- 
tivity of  the  eye  for  green  light  is  much  greater  than  for  red 
light,  the  difference  is  far  greater  for  low  than  for  high 
illumination,  that  is,  the  ratio  of  sensitivity  for  green  com- 
pared with  that  for  red  is  greater  for  faint  illumination  than 
for  intense  illumination.  If  you  desire  to  express  lights  of 
different  colors  in  candle  power  it  therefore  seems  necessary 


LIGHT  AND  ILLUMINATION  239 

also  to  give  the  distance,  or  the  intensity  of  illumination  at 
which  you  have  observed ;  in  other  words,  the  light  from  the 
middle  and  the  short  wave  end  of  the  spectrum  gives  a  better 
and  more  efficient  illumination  where  the  total  intensity  of 
illumination  is  low,  while  the  long  wave  or  low  frequency  of 
the  red  and  orange  and  yellow  light  gives  a  much  more  bril- 
liant effect  at  high  intensity  than  the  same  volume  of  light  of 
shorter  wave  length. 

This  is  of  importance  for  the  illuminating1  engineer, 
because  where  you  desire  to  get  high  intensity  effects,  as  in 
decorative  lighting  or  in  advertising,  better  results  are  given 
by  the  low  frequency  end  of  the  spectrum,  by  orange  and 
yellow  light,  whereas  when  you  are  satisfied  with  low  intensity 
of  illumination,  as  in  street  lighting,  you  get  better  results  from 
the  short  wave  end  or  the  middle  of  the  spectrum,  from  the 
greenish-yellow  of  the  Wellsbach  gas  light  and  the  bluish- 
green  of  the  mercury  lamp,  and  not  from  the  orange-yellow  of 
the  old  incandescent  lamp.  Therefore  the  white  light  of  the 
carbon  arc  gives  better  results  in  street  lighting  than  the  yel- 
low of  the  incandescent  lamp,  even  at  equal  intensity  of 
illumination.  These  features  have  been  of  less  importance 
until  a  few  years  ago,  since  the  available  sources  of  light  were 
all  approximately  of  the  same  color,  varying  from  the  orange- 
yellow,  to  yellow  and  yellow-white,  to  white;  from  the  gas 
lamp,  kerosene  lamp  an*d  tallow  candle  of  orange-yellow  color, 
to  .the  yellow  incandescent  lamp  and  the  yellowish-white  arc, 
yellowish-white  sunlight,  to  the  white  diffused  daylight.  This 
was  a  fairly  limited  range.  It  is  only  in  the  last  few  years  that 
illuminants  of  high  efficiency  have  been  brought  out,  which 
give  marked  and  decided  color  differences,  and  are  available  in 
units  of  suitable  size  and  of  high  efficiency,  as  the  greenish- 
yellow  of  the  Wellsbach  gas  lamp,  the  bluish-green  of  the  mer- 


240  GENERAL  LECTURES 

cury  lamp,  and  the  orange-yellow  of  the  flaming  arc,  and  hence 
these  questions  are  increasing  in  importance. 

II. 

This  brings  us  to  the  consideration  of  the  methods  of 
producing  light.  Until  a  few  years  ago,  until  the  develop- 
ment of  the  Wellsbach  gas  mantle,  practically  all  methods  of 
producing  light  were  based  on  incandescence.  That  is,  by 
impressing  energy  on  a  solid  body,  either  the  chemical  energy 
of  combustion,  or  electric  energy  in  the  incandescent  or  car- 
bon arc  lamp,  the  temperature  is  raised  to  such  a  high  degree 
that  amongst  the  total  radiation  issuing  from  the  heated  body 
a  certain  very  small  percentage  appears  within  the  fraction  of 
an  octave  of  visible  light.  With  increasing  temperature  of  the 
radiating  body,  the  average  wave  length  of  radiation  decreases, 
that  is,  the  average  frequency  of  radiation  increases  and  so 
approaches  nearer  to  the  visible  range,  although  still  at  the 
very  highest  temperature  which  can  be  produced  the  average 
wave  length  of  radiation  is  very  far  below  the  visible.  This 
means  that  the  higher  a  temperature  is  reached  by  an 
incandescent  body,  the  higher  is  the  average  frequency  of 
radiation,  and  therefore  the  larger  a  percentage  of  the  total 
energy  of  radiation  is  within  the  visible  range,  as  light.  The 
problem  of  efficient  light  production  by  incandescence  therefore 
is  the  problem  of  reaching  as  high  a  temperature  as  possible  in 
the  luminous  body.  In  the  gas  flame  and  the  kerosene  lamp, 
this  temperature  is  the  temperature  of  combustion,  rather  lim- 
ited. In  the  incandescent  lamp  it  is  limited  also.  In  the  latter 
case  the  temperature  which  can  be  reached  is  limited  by  self- 
destruction  of  the  incandescent  body. 

The  highest  temperature  probable  which  can  be  reached 
is  the  boiling  point  of  carbon ;  it  is  reached  in  the  crater  of  the 


LIGHT  AND  ILLUMINATION  241 

carbon  arc  lamp,  and  therefore  the  carbon  arc  gives  the  most 
efficient  incandescent  light.  It  is  incandescent  light,  because 
it  conies  from  the  incandescent  crater,  and  the  arc  flame  or  the 
vapor  conductor  does  not  appreciably  contribute  to  the  amount 
of  light  issuing  from  the  arc  lamp.  Very  much  lower,  neces- 
sarily, is  the  temperature  of  the  incandescent  lamp,  of  the  car- 
bon filament. 

The  problem  is  to  find  materials  which  can  stand  very 
high  temperatures,  to  increase  the  temperature  of  the  gas  flame 
as  well  as  of  the  incandescent  filament.    We  have  increased  the 
temperature  of  the  gas  flame  by  using  a  gas  of  higher  chemi- 
cal energy,  as  acetylene.     The  acetylene  flame  is  white;  the 
ordinary  gas  flame  is  yellow.    We  have  increased  the  tempera- 
ture of  the  carbon  filament  by  replacing  the  carbon  with  some 
more  refractory  material,  such  as  tantalum,  osmium,  tungsten, 
etc.,  and  thus  getting  a  higher  efficiency.   We  can  increase  the 
temperature  of  the  gas  flame  by  increasing  the  rapidity  of  com- 
bustion.   We  can  increase  the  temperature  of  the  carbon  fila- 
ment in  the  incandescent  lamp  by  increasing  the  energy  input, 
but  if  we  increase  the  temperature  of  the  carbon  filament,  it  is 
more  rapidly  destroyed.    If  we  increase  the  temperature  of  the 
gas  flame  by  more  rapid  combustion — to  a  certain  extent  we 
have  done  it  already,  by  having  the  gas  issuing  not  from  a 
round  hole,  but  from  a  flat  slit,  so  as  to  give  a  larger  surface 
to  the  flame;  if  we  go  still  further  and  mix  the  gas  with  air, 
we  get  a  still  higher  temperature,  a  more  rapid  combustion, 
but  we  loose  the  incandescent  body,  because  the  incandescence 
of  the  gas  flame  is  the  light  given  by  carbon  or  heavy  hydro- 
carbon particles,  floating  in  the  gases  of  combustion.     We 
could  increase  the  efficiency  of  the  gas  flame  by  mixing  the 
gas  with  air,  as  in  the  Bunsen  flame,  but  we  have  then  to  insert 
a  luminous  body  of  some  other  material,  as  no  carbon  is  pro- 


242  GENERAL  LECTURES 

duced  by  the  gas  in  its  dissociation.  We  can  do  it  by  a  skele- 
ton of  platinum  wire.  In  no  case,  however,  can  we  reach  very 
high  efficiencies  by  incandescence,  due  to  the  temperature  limit. 

We  could,  however,  increase  the  efficiency  of  light  pro- 
duction if  we  could  find  an  incandescent  body  which  would  not 
radiate  in  the  same  manner  as  the  carbon  filament  or  the  so- 
called  black  body,  but  which  would  give  an  abnormally  low  rad- 
iation in  the  low  frequency  range,  or  an  abnormally  high  radia- 
tion in  the  high  frequency  or  visible  range.  Such  a  body  may  be 
said  to  give  selective  radiation,  because  the  distribution  of  ener- 
gy in  the  spectrum  amongst  the  different  frequencies  of  radi- 
ation is  not  the  same  as  it  would  be  with  an  ordinary  black  body 
of  the  same  temperature.  If  we  found  a  body  which  would  give 
an  abnormally  low  radiation  in  the  visible  range,  or  abnormally 
high  radiation  in  the  invisible  range,  this  body  would  be  an 
abnormally  inefficient  light  producer.  Vice  versa,  if  we  found  a 
body  giving  abnormally  high  radiation  of  short  wave  lengths, 
in  the  visible  range,  or  abnormally  low  radiation  of  long 
waves,  of  low  frequency,  this  would  give  an  abnormally 
efficient  incandescent  body.  Such  bodies  exist  and  the  enor- 
mous progress  in  gas  lighting  made  by  the  introduction  of  the 
Wellsbach  mantle  is  based  on  selective  radiation,  that  is,  the 
oxides  do  not  radiate  the  same  range  and  intensity  of  waves 
as  a  black  body,  the  incandescent  carbon,  but  give  an  abnorm- 
ally large  amount  of  visible  rays  compared  with  invisible  rays ; 
that  is  to  say, — they  give  a  larger  percentage  of  high  frequency 
light  rays  compared  with  the  low  frequency  invisible  rays. 
Possibly  and  even  probably  some  of  these  highly  efficient  fila- 
ments like  the  tungsten  filament,  also  owe  some  of  their  high 
efficiency  of  one  watt  per  candle  power  to  selective  radiation. 

When  discussing  selective  radiation,  we  have  first  to 
come  to  an  agreement  on  what  we  understand  by  selective 


LIGHT  AND  ILLUMINATION  243 

radiation.  The  question  whether  an  illuminant  owes  its  high 
efficiency  to  selective  radiation,  depends  largely  on  the  defini- 
tion of  the  term  "selective  radiation".  We  have  here  a  simi- 
lar case  to  that  of  the  much  discussed  problem  of  the  "counter 
electromotive  force  of  the  electric  arc".  Whether  the  electric 
arc  has  a  counter  e.  m.  f .  or  not,  entirely  depends  on  the  defini- 
tion of  counter  e.  m.  f.  In  the  same  way,  the  decision  on  the 
question  of  selective  radiation  depends  upon  what  you  define  as 
selective  radiation.  If  you  define  as  selective  radiation  any 
radiation  in  which  the  intensity  of  radiation  is  distributed 
through  the  total  spectrum  differently  from  that  of  the  theoreti- 
cal black  body,  then  the  Wellsbach  mantle  has  selective  radia- 
tion. If,  however,  you  define  selective  radiation  as  the  radia- 
tion of  a  body  which  gives  spectrum  lines,  or  bands,  or  absorp- 
tion lines  and  bands,  that  is,  sharply  defined  narrow  ranges  in 
the  spectrum,  of  abnormally  high  or  abnormally  low  intensity, 
then  the  Wellsbach  mantle  has  no  selective  radiation.  So  all 
discussions  and  statements  on  selective  radiation  have  rather 
little  meaning,  if  the  writer  does  not  give  his  definition  of 
selective  radiation.  In  the  following,  I  define  as  selective, 
any  radiation  which  differs  in  the  distribution  of  its  intensity 
from  the  radiation  of  the  theoretical  black  body. 

In  an  incandescent  lamp  filament  we  do  not  get  a  definite 
pitch,  or  definite  frequency  of  vibration,  but  we  get  an  infinite 
number  of  different  waves.  The  reason  is  perhaps,  that  in  a 
solid  or  liquid  body  the  vibrating  particles  are  so  close  together 
as  to  interfere  with  each  other.  If  you  could  set  a  body  in 
vibration,  in  which  the  vibrating  particles,  atoms  or  molecules, 
are  so  far  apart  as  not  to  interfere  with  each  other,  as  in  a  gas 
at  low  pressure,  then  they  would  execute  their  own  periods  of 
vibration,  and  then  .the  light  from  such  a  body  would  not  be  a 
radiation  of  all  wave  lengths,  but  we  would  get  radiations  of 


244  GENERAL  LECTURES 

only  a  few  definite  wave  lengths,  or  a  line  spectrum.  So  in- 
candescent or  luminous  sodium  vapor  gives  only  one  kind  of 
light,  a  yellow  spectrum  line,  and  in  addition  thereto  a  number 
of  utrared  and  ultraviolet  rays. 

Since  the  spectrum  light  is  based  on  the  non-interference 
of  the  vibrating  particles,  it  is  easy  to  understand,  that  when 
you  bring  tthe  atoms  or  molecules  closely  together — as  at 
atmospheric  pressure — interference  may  begin,  and  the  lines 
of  the  spectrum  become  more  confused  and  blurr  into  bands. 
Therefore,  we  see  in  the  mercury  arc  spectrum,  which  is  at 
low  vapor  pressure,  a  small  number  of  definite,  sharply  definite 
lines.  In  the  calcium  spectrum  of  the  flame  carbon  arc,  we 
get  a  large  number  of  lines  blurring  into  each  other  to  an 
almost  continuous  spectrum;  so  also  in  the  white  spectrum  of 
the  magnetite- titanium  arc. 

When  we  set  a  gas  or  vapor  in  vibration,  it  vibrates  at 
its  own  frequency,  independent  of  the  temperature,  and  it  is 
merely  a  question  of  the  character  of  the  material  whether  a 
very  large  percentage  of  the  total  energy  of  radiation  happens 
to  be  within  the  visible  range  or  outside  the  visible  range. 
Temperature  does  not  come  in  as  factor,  because  the  frequency 
of  radiation  is  no  longer  a  function  of  the  temperature,  but 
independent  of  the  temperature.  Sodium  vapor  gives  the  same 
frequency  of  radiation,  the  same  yellow  line  when  the  sodium 
vapor  is  at  low  temperature  or  at  high  temperature.  Some 
spectrum  lines  may  increase  in  intensity  with  an  increase  of 
temperature  faster  than  others,  and  the  color  of  light  may 
change  with  the  -temperature,  change  from  yellow  to  white  or 
blue,  or  from  green  toward  white,  and  red,  as  the  mercury 
light  does  with  increasing  temperature,  but  that  is  merely  a 
characteristic  feature  of  that  particular  body,  and  not  a  general 
character  of  the  temperature  effect;  the  possibility  there- 


LIGHT  AND  ILLUMINATION  245 

fore  exists  of  finding  materials  which,  when  energized,  as 
vapor  or  gas,  give  a  spectrum  with  a  large  amount  of  energy 
in  the  visible  range,  thus  giving  an  efficiency  of  light  produc- 
tion far  in  excess  of  that  available  by  incandescence. 

So  far  the  only  materials  which  give  these  characteristics 
are  mercury,  calcium  and  titanium.  These  three  metals  give 
spectra  which  contain  such  a  large  percentage  of  the  total 
radiation  in  the  visible  range,  that  the  amount  of  light  meas- 
ured physiologically  in  candle  power  is  far  in  excess  of  that 
which  possibly  could  be  produced  by  incandescence,  even  with 
the  assistance  of  selective  radiation.  Their  industrial  appli- 
cations are  represented  by  the  mercury  arc,  the  yellow  flame 
carbon,  and  the  white  magnetite  and  titanium  arc,  and  these 
are  of  such  very  high  efficiency  as  to  be  of  higher  magnitude 
than  any  incandescent  light. 

Even  if  we  consider  only  these  three  illuminants,  we 
have  quite  a  color  scale.  From  the  orange-yellow  of  the  flame 
carbon,  which  is  about  the  longest  wave  length  we  could  use, 
to  yellow  and  yellow-white,  in  the  acetylene  flame  and  the 
tungsten  filament.  Then  we  have  the  greenish-yellow  of  the 
Wellsbach  mantle,  by  selective  radiation.  We  have  the  bluish- 
green  in  the  mercury  arc,  and  the  yellowish-white  of  the  car- 
bon arc,  as  well  as  the  clear  white  of  the  titanium  arc.  Each 
of  these  can  be  modified.  We  can  modify  the  titanium  arc, 
giving  all  colors  from  yellow-white  to  bluish-white,  by  the 
addition  of  other  materials  which  give  either  yellowish  or 
bluish  spectra.  We  can  modify  the  yellow  calcium  arc,  from 
the  orange-yellow  of  calcium  fluoride  down  to  the  yellowish- 
white  of  calcium  borates.  You  can  modify  each  color  over  a 
certain  range,  and  you  can  get  pretty  nearly  any  color,  with 
the  exception  perhaps  of  a  clear  blue  and  violet :  no  means  have 
been  found  to  get  approximately  the  same  efficiency  in  those 


246  GENERAL  LECTURES 

colors  of  very  short  wave  length,  as  in  the  other  colors  of 
lights. 

This  feature  makes  the  effect  of  color  which  I  discussed 
before,  the  variation  of  the  physiological  effect  with  the  bril- 
liancy of  illumination,  of  more  importance  now  than  years 
ago,  when  the  only  method  of  producing  colored  light  was  by 
the  absorption  of  all  other  colors. 

III. 

After  all,  however,  it  is  not  light,  that  is  wanted,  but 
illumination ;  it  is  not  the  amount  of  visible  rays  issuing  from 
the  source  of  light,  the  incandescent  lamp  or  gas  flame,  which 
is  of  importance,  but  the  amount  of  light  which  reaches  the 
objects  we  desire  to  see,  that  is,  the  illumination  produced  by 
light.  In  this  respect,  I  believe  a  mistake  has  been  made  by  the 
gas  industry  as  well  as  the  electric  lighting  industry,  for 
many  years,  by  devoting  all  energy  to  the  production  of  light, 
the  development  of  the  lamp,  while  they  have  almost  entirely 
left  out  of  consideration,  that  the  production  of  an  efficient  light 
is  not  the  only  important  problem,  but  that  of  the  same  import- 
ance is  the  arrangement  of  the  light  so  as  to  get  efficient  illum- 
ination, that  is,  get  the  greatest  benefit  from  the  light  produced ; 
and  this  feature  has  been  usually  left  to  the  tender  mercies 
of  the  architect  or  the  decorator,  who  placed  the  lights  where- 
ever  he  thought  they  would  look  artistic,  regardless  of  the 
requirements  of  effective  illumination.  If  you  look  around, 
you  find  cases  everywhere  of  artificial  illumination  where  the 
lights  have  been  arranged,  so  that  you  get  a  very  poor  illumin- 
ation from  a  large  amount  of  light.  To  overcome  these 
defects,  it  is  necessary  to  study  the  problems  involved  between 
the  production  of  light  and  the  physiological  effect  produced 
by  the  light  upon  the  eye,  and  it  requires  a  careful  study,  just  as 


LIGHT  AND  ILLUMINATION  247 

any  other  engineering  problem.  It  is  of  importance  to  con- 
sider not  only  the  amount  of  light  issuing  from  the  source,  but 
the  amount  of  light  which  reaches  the  object  to  be  seen  by  the 
illumination. 

The  demands  of  illumination  are  mainly  of  two  classes, 
general  illumination,  and  local,  or  concentrated  illumination. 
Many  cases  require  general  illumination,  as  a  meeting  room, 
where  it  is  desired  to  see  equally  well  everywhere;  that  is,  to 
get  the  same  intensity  of  illumination  throughout  the  whole 
illuminated  area.  So  also  a  draughting  room,  a  school  room, 
the  hall  of  a  house  and  the  streets  of  a  city  require  general 
illumination,  a  uniform  fairly  high  intensity  in  a  draughting 
room  or  school  room,  a  relatively  low  but  as  nearly  as  possible  a 
uniform  intensity  in  the  streets  of  a  city.  It  is  true,  street 
lighting  is  usually  very  far  from  uniform,  but  that  merely 
means  that  the  problem  of  proper  street  lighting  is  usually  not 
solved  in  the  most  efficient  and  satisfactory  manner.  In  other 
cases  concentrated  lighting  is  required,  as  in  domestic  lighting, 
in  the  dining  room,  the  living  room,  etc.,  where  light  is  desired 
on  the  table  where  we  work,  eat,  read,  etc.  In  such  cases,  the 
general  illumination  of  the  room  is  of  lesser  importance ;  it  is 
not  needed  to  any  extent,  or  is  frequently  undesirable,  because 
a  room  with  a  very  low  intensity  of  general  illumination  fre- 
quently is  considered  more  homelike,  especially  by  the  feminine 
part  of  the  human  race.  In  still  other  places  general  illumina- 
tion may  be  directly  objectionable,  as  in  a  sick  room.  Most 
cases,  however,  require  a  general  illumination  of  moderate 
intensity,  and  a  far  more  intense  local  illumination,  as  over 
the  desks  in  an  office,  or  the  reading  tables  in  a  library.  In 
such  cases  merely  a  general  illumination  would  be  sufficient, 
if  very  intense,  but  -this  is  uneconomical  and  to  some  extent 
objectionable  on  account  of  the  blinding  glare,  which  is  disa- 


248  GENERAL  LECTURES 

greeable;  and  so  a  combined  general  and  local  illumination  is 
more  efficient  and  more  satisfactory. 

In  producing  illumination  either  direct  lighting  or  indi- 
rect lighting  may  be  used.  That  is,  the  rays  issuing  from  the 
source  of  light  may  either  pass  directly  to  the  illuminated 
objects,  or  they  may  pass  to  a  reflecting  surface,  and  be 
reflected  from  this  surface  to  the  object,  or  may  pass  through 
a  refracting  body,  as  the  frosted  incandescent  lamp  globe,  or 
opal  globe  of  the  arc  lamp,  and  so  reach  the  illuminated  object. 
In  general,  it  is  obvious  that  any  method  of  indirect  lighting 
by  refraction  or  reflection  wastes  a  considerable  amount  of 
light.  That  means,  the  total  amount  of  light  which  reaches  the 
illuminated  object  must  necessarily  be  less  with  indirect  light- 
ing, as  compared  with  direct  lighting,  with  the  same  amount  of 
light. 

Indirect  lighting  can  be  done  by  reflection  or  refraction 
by  some  attachment  to  the  lamp,  as  a  reflector  or  a  holophane 
or  frosted  globe,  or  by  reflecting  the  light  from  the  ceilings  and 
walls  of  the  room,  on  the  objects  -to  be  illuminated.  In  the 
latter  case,  it  is  obvious  that  white  walls  give  the  highest 
efficiency  of  reflected  light.  It  is  easy  to  observe  that  the  same 
source  of  light  in  a  room  with  white  walls  gives  several  times 
the  intensity  of  illumination  which  it  gives  in  a  room  with 
black  or  non-reflecting  walls.  That  means  that  (the  total  amount 
of  illumination  is  increased  several-fold  by  reflection  from 
white  walls.  So  in  a  draughting  room,  or  school  room,  by 
using  as  light  walls  as  possible,  we  get  the  best  efficiency  of 
illumination. 

It  is  not  always  feasible  to  have  light  walls,  especially 
wrhen  you  come  to  machine  shops  or  foundries,  and  other 
places  where  the  walls  do  not  remain  white,  but  change  to 
some  darker  color.  The  question  is,  what  color  do  these  walls 


LIGHT  AND  ILLUMINATION  249 

assume?  The  color  of  almost  everything  which  is  changed  by 
age  is  due  to  either  iron  or  carbon.  In  most  cases  of  discolora- 
tion by  age  you  see  the  reddish-brown  color  of  iron  and  the 
brownish-yellow  color  of  carbon.  This  is  the  color  most  sub- 
jects gradually  assume.  This  color  of  age  is  in  the  long  wave 
or  low  frequency  end  of  the  spectrum.  To  get  the  benefit  of 
reflected  light  from  walls  which  cannot  be  kept  perfectly  white, 
a  source  of  light  rich  in  the  long  low  frequency  waves,  or  of  a 
yellowish  tinge  is  therefore  more  efficient  by  giving  more 
reflection  from  the  walls  than  a  source  of  light  rich  in  short 
or  high  frequency  waves,  that  is,  bluish-white.  This  effect  is 
very  marked  when  you  compare  the  mercury  lamp  with  the 
flame  carbon  lamp.  The  illumination  given  by  the  mercury 
lamp  in  a  draughting  room  is  very  satisfactory.  The  same 
illumination  in  a  foundry  or  machine  shop  is  far  less  satisfac- 
tory, and  you  notice  there  a  marked  absence  of  reflected 
light,  that  is,  the  walls  and  ceilings  all  gradually  assume  a 
color  which  is  rich  in  red  and  yellow,  and  so  reflect  very  little 
light  of  the  violet  end  of  the  spectrum.  Even  -the  black- 
begrimed  walls  of  a  blacksmith  shop  reflect  a  considerable 
amount  of  light  with  an  orange-yellow  source  of  light;  prac- 
tically none  with  the  bluish-green  of  the  mercury  lamp. 

Thus  the  shade  of  color  of  the  illuminant  may  be  very 
essential  in  getting  efficient  illumination.  In  the  interior  of  a 
city,  the  walls  usually  have  a  reddish-yellow  color.  In  that 
case  white  or  yellowish  lights  are  superior.  When  outside  of 
a  city,  the  greenish-yellow  of  the  Wellsbach  lamp,  the  bluish- 
green  of  the  mercury  arc,  gives  a  much  larger  amount  of 
reflected  light  from  vegetation  than  the  yellow  of  the  incandes- 
cent light  and  so  a  better  illumination.  Vegetation  absorbs 
the  long  waves,  the  low  frequency  radiation;  so  with  a 
yellow  source  of  light  there  is  practically  no  reflection  from 


25o  GENERAL  LECTURES 

living  vegetation,  but  only  reflection  from  dead  vegetation, 
and  in  the  light  of  the  incandescent  lamp  or  the  flame  arc  all 
vegetation  appears  very  poorly,  the  dead  parts  are  very  promi- 
nent, while  the  reverse  is  the  case  where  the  light  is  deficient 
in  the  red  and  yellow,  and  rich  in  the  green  and  blue,  as  with 
the  mercury  arc :  the  green  shows  prominently,  while  the  dead 
leaves,  etc.,  are  not  visible. 

IV. 

It  is,  however,  not  the  amount  of  light  which  reaches  the 
illuminated  objects  which  is  of  importance,  that  is,  not  the 
physical  intensity  of  illumination,  but  the  amount  of  light 
which  from  these  illuminated  objects  reaches  the  human  eye. 
With  the  same  intensity  of  illumination,  the  same  amount  of 
light  reaching  the  illuminated  object,  of  the  same  color,  the 
amount  of  light  entering  the  eye  may  vary  widely  with  the 
opening  or  contraction  of  the  pupil  of  the  eye.  The  eye  is 
automatically  adjusting  for  intensity  of  light.  This  is  the 
reason  we  see  well  at  sun  light  and  at  the  light  of  the  full  moon, 
although  the  former  is  many  thousand  times  greater  in  inten- 
sity than  the  latter.  The  eye  can  accommodate  itself  to  intens- 
ities varying  over  an  enormous  range.  It  does  this  partly  by 
the  fatigue  of  the  nerves  of  vision,  partly  by  the  contraction 
or  opening  of  the  pupil.  This  is  undoubtedly  a  protective 
device  developed  in  the  human  race.  It  means  that  if  we  have 
in  the  field  of  vision  a  source  of  light  of  high  intrinsic  bril- 
liancy, the  eye  protects  itself  by  contraction  of  the  pupil  and  so 
it  receives  very  much  less  light  in  the  field  of  vision  where  we 
want  to  see  objects,  than  if  the  source  of  light  were  taken  out 
of  the  field  of  vision.  By  eliminating  the  source  of  light  from 
the  field  of  vision  and  eliminating  the  contraction  of  the  pupils 
resulting  from  the  high  intrinsic  brilliancy  of  the  illuminating 


LIGHT  AND  ILLUMINATION  251 

body,  we  get  actually  a  much  larger  amount  of  light  into  the 
eye  with  the  same  amount  of  light  striking  the  illuminated 
object ;  that  is,  we  get  a  higher  physiological  efficiency.  Even 
with  a  much  smaller  amount  of  light  reaching  the  illuminated 
objects,  we  still  get  more  light  in  the  eye.  That  means  if  we 
reduce  the  intrinsic  brilliancy  of  the  illuminant  by  indirect 
lighting,  by  diffusing  the  light,  we  may  lose  a  considerable 
amount  of  light,  actually  get  a  considerably  reduced  quantity 
of  light  on  the  object  which  we  desire  to  see,  but  still  we  get 
a  larger  amount  of  light  from  these  objects  into  the  eye, 
because  the  eye  is  open  further  and  admits  more  light  and  is 
less  fatigued. 

It  follows  from  this  that  in  efficient  illumination,  it  is  of 
foremost  importance  to  arrange  the  illuminants  so  as  not  to 
have  excessive  intrinsic  brilliancies  in  the  field  of  vision  when 
looking  at  the  objects  we  desire  to  see.  That  means  that  the 
proper  field  for  the  illuminant  is  outside  of  the  field  of  vision, 
or  where  you  cannot  get  it  out  of  the  field  of  vision,  that  its 
intrinsic  brilliancy  should  be  reduced  by  diffusion :  thereby  we 
actually  get  a  much  higher  physiological  effect.  This  is  the 
reason  for  indirect  lighting.  We  may  have  a  very  large 
amount  of  light  thrown  on  any  object  in  a  room,  but  if  the 
eye  is  fatigued  by  seeing  the  source  of  light  in  the  field  of 
vision,  we  get  very  little  light  in  the  eye,  while  with  a  properly 
arranged  indirect  lighting,  with  a  much  lower  amount  of  light 
reaching  the  object,  we  get  a  higher  physiological  effect,  that 
is  a  better  and  more  efficient  illumination. 

It  appears,  however,  that  this  automatic  protective  faculty 
of  the  eye  was  developed  through  the  ages  as  a  protection  not 
against  light,  but  against  energy;  apparently  the  eye  is  pro- 
tecting against  the  energy  of  radiation,  not  the  physiological 
intensity,  and  since  the  energy  of  radiation  is  mainly  in  the 


252  GENERAL  LECTURES 

ultrared,  in  the  long  waves,  the  frequency  which  causes  the 
protective  reaction  is  the  frequency  of  the  long  wave  end  of 
the  spectrum,  the  red  and  yellow  waves;  they  make  the  pupil 
contract.  This  action  is  much  less  for  the  green  and  blue  rays. 
That  is  the  reason  the  eye  does  not  react  on  the  mercury  lamp 
to  any  great  extent.  It  means  a  green  light,  like  the  mercury 
or  Wellsbach  lamp,  can  be  in  the  field  of  vision  to  a  much 
greater  extent  without  causing  the  contraction  of  the  pupil  and 
so  reducing  the  physiological  effect.  This  is  of  importance  in 
places  where  the  light  cannot  well  be  taken  out  of  the  field  of 
vision,  as  in  street  illumination.  In  this  case,  all  the  sources 
of  light  must  be  arranged  along  the  street  and  so  must  be  in 
the  field  of  vision.  By  cutting  off  the  red  end  of  the  spectrum 
you  eliminate  the  contraction  of  the  pupil,  and  get  the  full 
benefit  of  the  light  between  the  illuminants,  while  with  a 
yellow  source  of  light,  as  with  the  incandescent  or  arc  lamp 
of  old,  you  do  not  get  the  benefit,  that  is,  the  physiological 
effect  of  the  illumination  by  a  green  illuminant  in  such  cases 
is  superior  to  that  by  a  yellow  illuminant,  the  illumination 
appears  brighter  and  more  uniform. 

A  light  devoid  of  red  and  yellow  rays  is  at  the  same  time 
the  safest  and  most  harmless,  and  also  the  most  harmful.  It 
is  the  safest  and  most  harmless,  and  gives  the  most  uniform 
illumination,  if  its  intrinsic  brilliancy  is  sufficiently  low  to  be 
below  the  danger  limit  of  energy  of  radiation,  but  it  is  harmful 
if  above  that,  because  the  eye  does  not  protect  itself  against  it, 
probably  because  these  lights  have  not  existed  throughout 
all  the  ages  when  this  protective  action  of  the  eye  was  devel- 
oped, and  sunlight  and  fire  were  the  only  sources  of  light,  both 
rich  in  red  rays.  This  accounts  for  the  rather  contradictory 
effect  observed,  that  green  or  blue  light,  as  the  Wellsbach 
mantle  or  mercury  lamp,  is  a  very  good  light  to  work  by, 


LIGHT  AND  ILLUMINATION  253 

superior  to  the  yellow  kerosene  lamp,  and  at  the  same  time 
there  is  some  suspicion  that  it  is  harmful  to  the  eye.  It  may 
well  be  that  where  it  is  of  very  high  intensity,  the  automatic 
protection  of  the  eye  is  not  sufficient  to  protect  with  such  light. 
Where  you  use  such  sources  of  light  you  can  get  the  benefit 
of  the  absence  of  the  contraction  of  the  pupil,  but  it  devolves 
upon  you  to  arrange  the  illumination  so  as  not  to  get  the  harm- 
ful effects  against  which  the  automatic  protection  of  the  eye 
fails.  That  means  all  these  lights  are  superior  for  illumination 
if  they  have  low  intrinsic  brilliancies,  but  somewhat  question- 
able if  they  have  extremely  high  intensities. 

V. 

It  is,  however,  not  even  the  amount  of  the  light  which 
enters  the  eye  which  is  of  importance  in  illumination,  but  the 
difference  in  the  amount  of  light.  If  in  the  illuminated  area 
the  light  were  of  uniform  intensity,  and  everything  of  the 
same  color,  we  would  see  nothing  but  a  glare  of  light.  The 
seeing  takes  place  by  a  difference  in  color,  and  difference  in 
intensity.  Difference  in  intensity  includes  shadows.  Shadows 
are  thus  an  essential  feature  in  seeing  things. 

Considering,  then,  the  seeing  by  shadows  and  seeing  by 
color  differences,  you  observe  that  by  this  feature  we  can 
divide  illumination  into  directed  and  diffused  illumination.  In 
diffused  illumination  light  comes  in  all  directions  with 
approximate  uniformity,  and  shadows  do  not  exist :  in  directed 
illumination,  shadows  exist.  In  some  cases  shadows  are 
objectionable,  and  in  other  cases  shadows  are  necessary  for 
clear  distinction,  and  diffused  illumination  in  such  cases  would 
not  be  satisfactory. 

As  regard  to  seeing  differences  in  color,  it  is  obvious  that 
where  definite  color  distinctions  are  required,  you  can  intensify 


254  GENERAL  LECTURES 

the  sharpness  of  vision  by  selecting  the  color  of  your  light  best 
suited  to  bringing  out  the  colors  desired.  Where  the  color 
conditions  you  want  to  distinguish  are  .those  due  to  age,  iron 
and  carbon,  then  the  light  which  is  deficient  in  red  and  yellow, 
which  therefore  shows  the  colors  given  by  iron  and  carbon,  as 
black,  gives  a  much  sharper  distinction,  and  the  mercury  lamp 
shows  blemishes  and  dirt  much  more  pronounced  than  the 
white  light.  Again,  the  sources  of  light  which  are  very  rich 
in  red  and  yellow  rays  show  these  colors  due  to  iron  and  car- 
bon very  much  less,  and  therefore  show  blemishes  or  a  slight 
amount  of  dirt  much  less,  soften  them;  and  where  the  color 
distinctions  are  those  due  to  these  two  most  prominent  ele- 
ments, in  the  yellow  light  their  appearance  will  be  greatly 
softened,  and  under  the  green  light  they  will  be  made  harsh 
and  sharp.  If  you  desire  to  soften  effects,  as  in  a  ballroom,  it 
would  be  fiendish  to  use  mercury  lamps,  but  where  you  want 
to  search  out  a  spot  that  is  soiled,  it  would  be  very  wrong  to 
use  a  dull,  yellow  incandescent  lamp  or  a  gas  flame,  but  rather 
to  use  the  green  Wellsbach  light,  or  better  still  the  bluish- 
green  mercury  arc,  which  gives  in  such  case  sharp  distinction, 
where  white  light  shows  little,  and  yellow  light  nothing. 
Where  you  desire  to  see  all  colors  in  about  the  same  relation  as 
by  daylight,  you  obviously  desire  a  white  light. 

It  is  therefore  important  for  the  illuminating  engineer 
to  select  the  shade  or  color  of  the  light  and  study  the  require- 
ments of  each  case  which  comes  into  his  charge.  It  would  be 
just  as  wrong  in  one  case  to  use  an  incandescent  lamp,  where 
the  mercury  lamp  would  be  better,  as  to  do  the  reverse. 

We  have  to  distinguish  then  between  general  illumina- 
tion and  local  illumination,  between  direct  illumination  and 
indirect  illumination,  and  between  directed  illumination  and 
diffused  illumination.  These  three  different  classes  or  distinc- 


LIGHT  AND  ILLUMINATION  255 

tion  to  a  certain  extent  overlap.  It  would  be  very  wrong,  how- 
ever, to  mistake  them,  and  a  very  serious  mistake  in  the  design 
of  a  system  of  illumination  can  very  easily  be  made;  for 
instance,  by  mistaking  general  illumination  and  'diffused 
illumination  for  each  other.  The  problem  may  be  to  get  uni- 
form intensity  all  over.  You  can  get  that  by  distributing  a 
large  number  of  small  units  all  around  the  cornices  and  reflect 
the  light  from  white  walls  and  ceilings  and  get  a  very  diffused 
illumination,  or  you  could  get  general  illumination,  where  the 
intensity  of  illumination  all  over  is  the  same,  in  a  moderate 
sized  room,  from  one  source  of  light  by  using  one  of  these 
sources  of  light  as  an  incandescent  lamp  with  a  holophane 
reflector,  which  gives  the  proper  distribution,  or  you  could 
get  light  from  any  other  source  by  controlling  the  distribu- 
tion curve  of  the  light  so  as  to  get  uniform  distribution.  The 
former  arrangement  gives  diffused  light,  the  latter  directed 
light.  You  may  get  -the  same  intensity  of  illumination  all  over 
the  room,  in  both  cases;  but  in  the  former  case  no  shadows, 
in  the  latter  case  absolutely  black  shadows.  Probably  in  the 
former  case  for  domestic  use  the  lighting  will  be  unsatisfac- 
tory and  trying  to  the  eyes,  because  you  do  not  see  well,  you 
do  not  have  any  shadows,  objects  around  you  are  not  so  dis- 
tinct, because  you  lose  the  distinguishing  feature  of  the 
shadow.  In  the  latter  case  with  the  directed  lighting  from 
one  source,  the  lighting  will  be  unsatisfactory  because  you  get 
very  dark  shadows,  and  you  do  not  see  anything  in  the 
shadows,  and  the  eyes  will  be  made  tired  by  trying  to  see  in 
the  very  dark  shadows. 

You  have  to  consider  how  much  directed  light  and  how 
much  diffused  light  you  require.  In  some  cases  you  may 
desire  only  diffused  light.  In  the  general  lighting  of  a 
draughting  room  you  do  not  want  any  directed  light,  since 


256  GENERAL  LECTURES 

you  must  have  no  shadows,  because  if  the  ruler  casts  a 
shadow,  it  is  trying  to  the  eyes  to  distinguish  between  the  edge 
of  the  ruler  and  the  edge  of  the  shadow,  and  mistakes  may  be 
made.  In  this  case,  you  see  only  by  differences  in  color  and  in 
the  intensity,  and  not  by  shadows.  You  therefore  get  satis- 
factory illumination  from  many  small  units,  or  by  indirect 
lighting,  reflected  light  from  white  walls  and  ceilings,  but  you 
get  unsatisfactory  illumination  from  a  few  units  even  when 
properly  distributed  so  as  to  give  uniform  intensity  all  over, 
but  giving  little  reflected  light.  In  other  cases,  you  may  also 
require  a  general  illumination  equal  in  intensity  all  over,  but 
you  need  directed  illumination  so  as  to  see  by  the  shadows.  So 
for  instance,  a  good  draughting  room  illumination  would  not 
be  suitable  for  a  foundry.  In  a  foundry,  where  all  the  objects 
assume  more  or  less  the  same  color,  you  require  shadows  to 
see  by.  Then  you  need  a  number  of  units  of  light  to  give 
directed  illumination,  but  you  must  not  go  so  far  as  to  be 
unable  to  see  in  the  shadows ;  you  must  have  some  diffusion,  or 
overlapping  of  the  different  beams  of  light.  So  if  you  take  a 
satisfactory  foundry  illumination  and  put  it  in  the  draughting 
room,  even  if  the  intensity  were  satisfactory,  it  would  be 
entirely  unsatisfactory,  and  so  would  be  the  reverse.  It  is 
therefore  not  merely  the  distribution  of  the  intensity  of  the 
light,  which  is  essential,  but  also  the  character,  whether 
diffused  or  directed  light,  or  how  divided  between  diffused  and 
directed  light. 

In  the  different  lighting  problems  you  therefore  meet  (the 
question  of  concentrated  and  general  illumination,  of  directed 
and  diffused  illumination.  In  domestic  lighting,  by  reflected 
light  from  white  walls  and  ceilings  we  can  get  a  high  intensity, 
and  can  increase  the  illumination  several-fold  over  that  given 
directly  from  the  source  of  light,  such  as  the  incandescent 


LIGHT  AND  ILLUMINATION  257 

lamp  or  gas  flame.     Still  the  illumination  would  be  unsatis- 
factory and  tiring  to  the  eyes.  We  all  know  that  in  the  home  a 
room  with  white  walls  is  not  as  agreeable  as  one  with  darker 
walls.    We  say  we  have  too  much  light.    But  we  do  not  have 
too  much  light,  because  we  do  not  have  anywhere  near  the 
same  amount  of  light  as  we  get  during  the  daytime  out  of 
doors.    We  have  too  large  a  percentage  of  diffused  light.  The 
intensity  of  diffused  light  is  too  great  as  compared  with  the 
directed  light.    We  lose  the  shadows  and  that  is  tiring  to  the 
eyes.     The    problem     of     domestic     lighting     then     is,     to 
get    sufficient    directed    and    not    too    much    diffused    light- 
ing so  as  to  get  the  best  vision,  that  is,  to  get  sufficient 
shadows  to  see  by,  but  the  shadows  must  not  be  so  dark  as  to 
make  seeing  objects  in  the    shadows    tiring    to    the     eyes. 
During  the  daytime  we  get  directed   light  from  the   win- 
dows, diffused  light  reflected  from  the  walls.     To  get  the 
proper  proportion  between  directed  and  diffused  light,  fixes 
the  shade  of  the  walls,  and  in  general  we  have  to  use  walls  of 
somewhat  darker  color.     When  you  come  to  lighting  in  the 
evening,  with  a  source  of  light  like  the  incandescent  lamp  or 
gas  lamp,  sending  out  light  in  all  directions,  the  diffused  light 
compared  with  the  concentrated  or  directed  light  is  a  higher 
percentage  than  in  the  daytime  for  the  same  color  of  walls, 
partly  due  to  the  color  of  the  light,  which  is  yellow,  and  is  more 
reflected  from  the  walls,  largely,  however,  because  with  the 
daylight  through  the  window  the  directed  light  is  a  much 
larger  percentage  of  the  total  light  than  in  the  lamp,  where 
only  a  small  part  is  concentrated  light.     It  is  not  comfortable 
to  have  this  strong  diffused  light,  and  so  we  put  shades  on 
which  absorb  three-quarters  of  the  light,  but  which  give  us 
a  more  comfortable  illumination  in  the  room.     That  means 
waste,  however,  and  you  pay  for  light  which  you  do  not  use. 


258  GENERAL  LECTURES 

The  proper  illuminating  engineering  then  is  to  secure  the  cor- 
rect distribution  curve  of  the  source  of  light,  so  as  to  give  the 
desired  amount  of  concentrated  lighting  on  the  dining  or  read- 
ing table,  and  give  only  as  much  diffused  lighting  as  is  com- 
patible with  the  amount  of  direct  light  used,  to  see  in  the 
shadows.  The  problem  of  domestic  lighting,  from  the  illum- 
inating engineering  point,  is  to  determine  the  illumination 
over  the  entire  area,  and  also  the  character  of  illumination, 
whether  directed  or  diffused;  how  large  an  amount  of  light 
should  be  concentrated,  and  how  large  an  amount  should  be 
directed ;  then  the  question  of  colors  and  shades  also  comes  in 
as  an  important  factor,  as  was  discussed  before.  Practically 
nothing  has  yet  been  done  in  this  direction  systematically  and 
intelligently,  but  all  has  been  done  by  trial  which  at  the  best 
usually  means  producing  more  light  than  necessary,  and  throw- 
ing away  the  excess  of  diffused  light  by  absorption. 


APPENDIX  II 


LIGHTNING  AND  LIGHTNING  PROTECTION 

Paper  read  before  the  Annual  Convention  of  the 
National  Electric  Light  Association,  1907. 

Revised  to  date. 
I.     LIGHTNING  PHENOMENA  IN  THE  CLOUDS. 


T 


HE  first  man  who  attacked  the  problem  of  lightning  and 
lightning  protection,  a  century  and  half  ago,  was  our 
great  citizen,  Benjamin  Franklin.  He  gave  us  the 
lightning  rod,  which  is  now  universally  recognized  as  the 
most  effective  and  only  protective  device  for  isolated  points, 
as  steeples,  chimneys,  etc.  The  next  step  in  advance  was  made 
by  Faraday :  he  showed  that  in  the  interior  of  a  perfectly  con- 
ducting body  no  electric  disturbances  can  be  produced  by  out- 
side electric  forces.  This  led  to  the  most  effective  protection 
possible  against  lightning  or  electric  disturbances,  the  use  of 
a  grounded  metal  cage,  "Faraday's  cage",  enclosing  the  struc- 
ture which  is  to  be  protected,  whether  a  building  against 
lightning,  or  a  delicate  instrument  against  electric  fields. 

In  its  simplest  form,  Faraday's  cage,  applied  to  a  trans- 
mission line,  is  the  ground  wire  above  the  line,  and  the  pro- 
tection afforded  by  it  is  the  more  complete,  the  more  the  over- 
head ground  wires  represent  the  condition  of  an  enclosing 
cage  of  perfect  conductivity.  That  is,  a  system  of  wires  above 
and  on  the  sides  of  a  transmission  line  is  superior  to  a  single 
wire,  a  wire  of  high  conductivity  superior  to  a  small  iron 
wire.  Here  I  specially  desire  to  draw  attention  to  the 
second  requirement  of  the  Faraday  cage,  high  conductivity. 
Thus  it  is  not  sufficient  merely  to  have  any  kind  of  overhead 


260  GENERAL  LECTURES 

grounded  wire  regardless  how  small,  but  high  conductivity 
of  .the  grounded  conductor  is  essential  in  many  cases  of 
atmospheric  disturbances. 

In  the  last  ten  years,  transmission  voltages  have  crept 
higher  and  higher,  transformers  have  been  built  of  consider- 
able size,  of  still  higher  voltages,  so  that  exact  data  on  the 
action  of  voltages  up  to  300,000  are  now  available,  and 
approximate  data  for  potentials  above  a  million  volts.  It  was 
found  that  air  has  a  definite  and  fixed  breakdown  strength, 
that  is,  just  as  a  beam  breaks  mechanically  as  soon  as  the 
stress  in  it  exceeds  a  definite  value,  the  breaking  strength  of 
the  material,  so  air  breaks  down  by  a  disruptive  spark,  as  soon 
as  the  electric  stress  in  the  air,  or  the  potential  gradient, 
exceeds  a  certain  value,  which  is  about  100,000  volts  per  inch. 
The  disruptive  strength  of  air  is,  over  a  wide  range,  propor- 
tional to  the  pressure,  that  is,  at  a  pressure  of  two  atmospheres 
it  is  .twice  as  high,  or  200,000  volts  per  inch;  at  one-quarter 
atmosphere  it  is  one-quarter,  or  25,000  volts  per  inch.* 

The  striking  distance  in  air  between  needle  points  has 
been  investigated  up  to  300,000  volts,  and  found  that  for  high 
voltages  it  is  very  nearly  10,000  volts  per  inch,  that  is,  a  dis- 
charge of  30"  length  between  needle  points  requires  300,000 
volts.  If  between  two  needle  points  the  potential  difference  is 
gradually  increased,  already  at  relatively  low  voltages  the  dis- 
ruptive strength  of  the  air  at  the  needle  points  is  exceeded,  the 
air  at  the  points  breaks  down  and  becomes  conducting,  and 
luminous,  as  "brush  discharge",  so  that  the  terminals  are  not 
the  needle  points  any  more,  but  the  whole  space,  of  approxi- 
mately spherical  shape,  which  is  covered  by  the  brush  dis- 
charge. As  result  thereof,  for  high  voltage,  no  appreciable 
difference  exists  in  the  striking  distance  between  needle  points 

*  Only  at  very  low  pressures,  where  the  distance  between  air  molecules  become  appreciable,  this  law 
ceasei,  and  the  disruptive  strength  increases  again,  and  seems  to  become  infinitely  great  in  a  perfect 
racnum. 


LIGHTNING  AND  LIGHTNING  PROTECTION    261 

and  between  spheres,  the  centers  of  which  approximately 
coincide  with  the  needle  points,  as  long  as  the  diameter  of  the 
spheres  is  small  compared  with  their  distance  apart  apart.  With 
increasing  potential  difference  between  needle  points,  the  brush 
discharges  spread  out  against  each  other,  until  only  about  40% 
of  the  space  between  the  needle  points  is  free,  and  then  a  dis- 
ruptive spark  passes. 

Naturally,  as  soon  as  determinations  of  spark  voltages 
became  available,  attempts  were  made  to  estimate  the  voltage 
of  a  lightning  flash.  Considering,  in  a  lightning  flash,  the  dis- 
charge as  that  in  an  ununiform  field,  similar  to  that  between 
needle  points,  and  so  requiring  about  10,000  volts  per  inch. 
In  this  case,  a  lightning  flash  of  two  miles,  or  about  10,000 
feet  length,  would  require  a  potential  difference  of  about  1200 
million  volts.  The  existence  of  such  voltages  in  the  clouds 
does  not  appear  possible:  a  potential  difference  of  1000  mil- 
lion volts  would  produce  a  brush  discharge  of  about  one-half 
mile  in  length,  before  the  final  lightning  flash  occurs.  In  the 
brush  discharge  the  air  is  electrically  broken  down,  and  becomes 
conducting.  But  it  is  also  mechanically  and  chemically  broken 
down,  that  is,  the  molecules  are  dissociated  and  recombine  after 
the  discharge,  in  all  possible  combinations.  That  is,  we  get 
ozone  and  nitric  acid,  and  a  lightning  flash  produced  by  a 
thousand  million  volts  would  thus  be  followed  by  a  deluge  of 
nitric  acid.  This  fortunately  is  not  the  case. 

An  estimate  of  the  voltage  and  the  current  in  a  lightning 
flash  would  not  yet  give  the  energy,  if  the  duration  of  the  dis- 
charge is  not  also  known.  We  can,  however,  get  an  approxi- 
mate estimate  of  the  magnitude  of  the  energy  of  the  lightning 
flash  indirectly,  from  photometric  considerations,  and  elimi- 
nate the  consideration  of  the  duration  of  the  flash  by  the 
integrating  feature  of  the  human  eye  for  impressions  of  very 


262  GENERAL  LECTURES 

short  duration:  an  impression  on  the  human  eye  persists  for 
some  time,  about  .1  seconds,  and  any  phenomenon  of  shorter 
duration  than  .  i  seconds  so  appears  to  last  .  I  seconds.  Hence 
the  effect  on  the  eye  by  a  lightning  flash  would  be  about  the 
same  whether  the  flash  lasted  .1  seconds,  or  if  it  were  of  a 
thousand  times  greater  intensity  but  lasting  a  thousandth  of  the 
time.  This  means  that  the  eye  would  see  a  lightning  flash 
about  in  the  same  manner  as  if  its  light,  and  so  probably  its 
energy  were  spread  uniformly  over  .  i  seconds. 

The  illumination  given  by  a  brilliant  lightning  flash  is 
about  of  the  same  magnitude  as  good  artificial  illumination, 
perhaps  one  foot  candle,  since  at  night  time  in  a  well  lighted 
room,  the  light  of  a  lightning  flash  is  still  quite  appreciable. 
Estimating  roughly  one  watt  per  candle  foot,  a  lightning  flash 
illuminating  a  space  of  two  miles  square  or  io8  square  feet, 
with  one  foot  candle  would  consume  io8  watts,  and  as  this  is 
the  illumination  as  averaged  by  the  human  eye  over  .  i  seconds, 
the  energy  is  io7  watt-seconds,  or  10,000  K.  W.  seconds. 
The  energy  of  a  large  lightning  flash,  estimated  from  its  light, 
would  thus  be  of  the  magnitude  of  10,000  K.  W.  seconds.  This 
value,  while  considerable  when  expressed  in  electric  quanti- 
ties, is  by  no  means  so  very  great:  reduced  to  heat  measure, 
it  only  equals  the  latent  heat  of  evaporation  or  condensation 
of  about  9  Ibs.  of  water. 

As  seen,  an  estimation  of  the  voltage  of  the  lightning 
flash  from  length  and  disruptive  potential  gradient  of  the 
air,  does  not  give  reasonable  values,  that  is,  the  lightning 
flash  cannot  be  a  single  discharge  as  that  of  a  Leyden  jar. 
An  estimation  of  the  voltage  may  then  be  attempted  in  a  differ- 
ent manner. 

Lightning  flashes  usually  occur  within  thunder  clouds 
and  only  rarely  from  cloud  to  cloud  or  from  cloud  to  ground. 


LIGHTNING  AND  LIGHTNING  PROTECTION    263 

They  therefore  seem  to  be  rather  due  to  equalization  of 
potential  differences  within  the  cloud,  than  to  dicharges  between 
oppositely  charged  bodies.  Lightning  occurs  mainly  when 
rapid  condensation  of  moisture  takes  place  in  the  air  and  the 
electric  phenomena  seem  ,to  be  the  more  intense,  the  greater 
the  rapidity  of  condensation,  or  rain  formation.  Thus  the 
atmospheric  electric  disturbances  seem  to  be  connected  with 
the  condensation  of  water  vapor  to  clouds  and  rain. 

There  exists  normally  a  potential  gradient  in  the  air. 
That  is,  a  potential  difference  exists  between  the  air  at  different 
elevations,  reaching  sometimes  several  hundred  volts  per  foot, 
so  that  we  can  estimate  as  a  fair  average,  a  natural  potential 
gradient  in  the  air,  in  vertical  direction,  of  about  100  volts  per 
foot.  A  point  100  feet  above  ground  may  show  a  potential 
difference  of  about  10,000  volts  against  ground.  Usually  the 
higher  strata  of  the  air  are  positive  against  the  lower.  The 
cause  of  this  potential  gradient,  whether  terrestrial  or  cosmic, 
is  of  no  interest  to  us  here,  but  merely  its  existence. 

It  is  of  interest  to  investigate,  what  effect  must  be 
expected,  from  our  well-known  physical  laws,  from  the  con- 
densation of  moisture,  and  agglomeration  of  the  moisture 
particles  to  rain  drops,  in  an  atmosphere  having  such  a  poten- 
tial gradient. 

Assuming  water  vapor  in  a  higher  stratum  of  the 
atmosphere  to  condense  to  moisture  particles,  these  moisture 
particles  have  the  potential  of  the  air  in  which  they  float,  that 
is,  have  a  considerable  potential  difference,  perhaps  hundred 
thousands  of  volts,  against  ground,  and  so  contain  an  electric 
charge  against  ground.  These  moisture  particles  conglomer- 
ate with  each  other  to  larger  moisture  particles  and  ultimately 
rain  drops.  By  the  collection  of  n3  particles  into  one,  the 
diameter  of  the  particle  has  increased  n  fold.  Its  capacity 


264  GENERAL  LECTURES 

has  also  increased  n  fold  (the  capacity  of  a  sphere  being  pro- 
portional to  the  diameter).  The  particle  contains,  however, 
the  accumulated  charges  of  n8  smaller  particles,  and  n3  times 
the  charge,  with  n  times  the  capacity,  gives  n2  times  the  poten- 
tial. It  follows  herefrom  that  with  the  conglomeration  of  the 
water  particles,  their  potential  must  increase  rapidly,  propor- 
tionately to  the  square  of  their  diameter.  The  conglomeration  of 
moisture  particles  in  the  clouds  is,  however,  very  uneven,  due 
to  the  uneven  distribution  of  moisture,  as  is  plainly  seen  by 
looking  at  any  cloud :  dense  or  dark  parts  representing  consid- 
erable condensation  and  so  considerable  moisture  content, 
alternate  with  light  parts,  in  which  little  or  no  condensation 
occurs.  As  a  result  thereof,  starting  with  a  uniform  potential 
in  the  stratum  of  the  air,  where  condensation  begins,  differ- 
ences of  potential  distribution  by  necessity  result  from  the 
differences  in  the  condensation  of  water  vapor  to  moisture  and 
the  accumulation  of  the  moisture  particles  to  larger  ones,  that 
is,  the  denser  portions  of  the  cloud  are  at  a  higher  potential 
than  the  lighter  portions.  Thus,  starting  with  uniform  poten- 
tial, and  thus  zero  potential  gradient  in  the  air  at  the  moment  of 
the  beginning  of  condensation,  potential  differences  and  thus 
potential  gradients  appear. 

Such  potential  differences  in  the  clouds  increase  with 
increasing  agglomeration  of  moisture  particles  to  rain  drops, 
and  so  the  potential  gradient  rises.  Assuming  even  as  low  a 
potential  gradient  as  100  volts  per  foot  in  the  cloud  at  the 
beginning  of  agglomeration  of  moisture  particles,  the  collec- 
tion of  n3  such  particles  to  one  rain  drop  of  n  times  the  diameter 
and  so  n  times  the  capacity,  but  containing  the  static  charge  of 
n3  particles,  gives  n2  times  the  potential,  and  since  the  dis- 
tances between  the  particles  are  now  n  times  as  large,  the 
potential  gradient  has  increased  n  fold.  That  is,  by  conglom- 


LIGHTNING  AND  LIGHTNING  PROTECTION    265 

eration  of  water  particles,  the  potential  gradient  rises  propor- 
tionately to  the  diameter  of  the  particles.  Estimating  then  the 
average  diameter  of  moisture  particles  as  icr4  inches  at  the  be- 
ginning of  agglomeration,  when  the  potential  gradient  in  the 
cloud  is  about  100  volts  per  foot,  then  the  breakdown  potential 
of  the  air,  of  between  100,000  and  200,000  volts  per  foot, 
would  be  reached  when  the  drops  have  reached  about  .1  to  .2 
inches  diameter,  that  is,  the  size  of  rain  drops. 

Potential  gradients  in  the  cloud  thus  gradually  rise,  until 
somewhere  the  disruptive  strength  of  the  air  is  reached,  and  a 
discharge  passes,  equalizing  the  voltage  at  this  spot.  This, 
however,  causes  a  greater  potential  gradient  at  the  ends  of  the 
discharge,  exceeding  the  breakdown  strength  of  the  air,  and 
so  causes  a  second  discharge,  following  partly  over  the  path 
of  the  first,  then  a  third  and  so  on,  until  all  of  the  potential 
differences  or  inequalities  of  the  potential  distribution  in  the 
cloud,  are  leveled  down  by  a  series  of  successive  discharges. 
The  phenomenon  thus  is  similar  to  that  of  a  landslide,  setting 
off  another  and  another  landslide.  Or  it  can  best  be  pictured 
by  representing  the  unequal  moisture  distribution  in  the  cloud 
by  a  relief  map  built  of  wet  sand,  the  dense  portion  of  the 
cloud,  and  therefore  the  portions  of  high  potential,  being  repre- 
sented by  the  hills,  the  light  or  low  potential  portions  of  the 
cloud  by  the  valleys  of  the  relief  map.  As  soon  as  (the  sand 
dries,  somewhere,  where  the  declivity  is  very  steep,  that  is,  the 
potential  gradient  is  very  high,  a  slide  occurs,  this  causes 
another  slide  and  so  on,  until  the  whole  configuration  of  sand 
settles  down  to  a  flat  and  smooth  shape,  the  hills  are  leveled  off 
and  the  valleys  filled. 

The  existence  of  such  successive  discharges,  following 
each  other  after  appreciable  intervals  of  time  in  the  same  path, 
has  been  shown  by  the  photographs  of  lightning  flashes  taken 


266  GENERAL  LECTURES 

with  a  rotating  camera.  In  this  case,  by  the  motion  of  the 
camera  the  successive  flashes  are  recorded  side  by  side,  and 
sometimes  more  than  forty  successive  discharges  have  been 
counted,  the  whole  phenomenon  lasting  about  .6  seconds,  that 
is,  quite  an  appreciable  time. 

Oscillographs  of  lightning  discharges  from  (dead)  trans- 
mission lines  also  showed  the  frequent  occurrence  of  multiple 
strokes,  or  strokes  following  each  other  within  a  fraction  of  a 
second. 

It  follows  herefrom,  that  lightning  flashes  in  the  clouds, 
of  several  miles'  length,  occur  without  any  considerable  poten- 
tial difference  between  the  ends  of  the  flash,  but  result  from 
the  disruptive  equalization  of  the  unequal  potential  distribu- 
tion in  the  clouds,  caused  by  unequal  vapor  density  and  so 
unequal  condensation  and  conglomeration  of  moisture  particles. 

This  also  explains  the  relatively  small  tendency  to  dis- 
charges between  cloud  and  ground,  across  a  space  in  which 
no  condensation  takes  place  and  so  no  unequal  potential  distri- 
bution supplies  the  power  of  the  discharge :  although  the  dis- 
tance between  cloud  and  ground  is  smaller  than  the  distance 
traversed  by  a  lightning  flash  in  the  clouds,  and  the  average 
potential  differance  between  cloud  and  ground  probably  is 
greater  than  -the  potential  differences  in  the  clouds,  a  discharge 
to  ground  probably  occurs  in  general  only  where  by  a  heavy 
downpour  of  rain  a  range  of  high  potential  is  carried  bodily 
part  ways  down  to  ground.  This  also  may  explain,  that  light- 
ning discharges  to  the  ground  are  usually  followed  by  a  heavy 
downpour  of  rain. 

The  potential  gradient  in  the  air  may  rise  to  disruptive 
values  in  still  another,  slightly  different  manner,  and  lead  to 
lightning  discharges  without  being  accompanied  or  followed  by 
rain.  By  conglomeration  of  moisture  particles  the  potential 


LIGHTNING  AND  LIGHTNING  PROTECTION    267 

gradient  rises,  as  described  above,  but  before  the  water  drops 
have  reached  sufficient  size  to  precipitate  as  rain,  evaporation 
again  sets  in:  for  instance  by  the  drops  falling  to  a  lower  and 
warmer  stratum  of  the  air,  or  by  intercepting  the  heat  of  the 
sun's  rays,  and  the  drops  thus  dwindle  away.  The  decrease  in 
size  of  the  drops  represents  a  decrease  of  capacity,  the  capacity 
being  proportional  to  (the  diameter,  and  as  each  drop  retains 
the  same  charge,  its  potential  increases  with  the  decrease  of 
size,  without  limit,  and  so  also  the  potential  gradient  until  its 
disruptive  value  is  reached  and  the  lightning  discharge  occurs. 
This  phenomenon  is  frequently  observed  towards  the  evening 
of  a  hot  summer  day,  and  is  called  "heat  lightning",  and,  being 
the  result  of  evaporation,  thus  does  not  lead  to  rain. 

Estimating  then  as  disruptive  strength  of  air  under  dis- 
charge conditions  in  a  non-uniform  field,  and  at  the  reduced 
air  pressure  in  the  clouds,  100,000  volts  per  foot,  the  average 
potential  gradient  in  the  path  of  the  lightning  discharge 
through  the  clouds  would  be  about  50,000  volts  per  foot.  This 
gradient,  however,  would  not  be  unidirectional,  but  the  poten- 
tial would  rise  from  a  low,  or  even  negative  value  at  a  light 
portion  of  the  cloud,  to  a  maximum  value  af  i  dense  position, 
then  decrease  again,  that  is,  give  a  gradient  in  opposite  direc- 
tion, to  a  light  position,  etc.,  and  the  potential  gradient  would 
vary  from  nothing  at  a  maximum  potential  point,  to  a  maxi- 
mum, equal  to  the  breakdown  strength  of  air  at  the  starting 
point  of  the  discharge,  to  zero  at  a  minimum  potential  point, 
etc. 

To  estimate  the  current  which  discharges  in  the  lightning 
flash,  the  conductivity  of  air  in  the  path  of  the  discharge,  and 
the  diameter  of  the  discharge  are  required,  and  as  both  are 
unknown,  any  estimate  must  be  very  approximate  only. 
The  specific  resistance  of  gases  and  vapors  decreases  with 


268  GENERAL  LECTURES 

increasing  temperature  and  with  decreasing-  pressure.  It  is  a 
few  ohm  centimeters  at  atmospheric  pressure  and  the  high 
temperature  of  the  magnetite  or  carbon  arc,  and  is  also  a  few 
ohm  centimeters  at  the  low  temperature  and  low  pressure  of  a 
high  current  Geissler  tube  discharge.  The  mercury  arc 
stream  also  gives  a  specific  resistance  of  a  few  ohms.  The 
temperature  of  the  air  in  the  lightning  discharge  probably  is 
moderately  high,  but  the  pressure  is  also  not  far  from  atmos- 
pheric, so  that  100  ohm  centimeters  may  not  be  very  far  from 
the  true  magnitude  of  the  resistance.  Estimating  one  to  two 
feet  as  the  diameter  of  the  discharge  path,  and  100  ohm  centi- 
meters as  the  specific  resistance,  and  allowing  for  the  induct- 
ance, gives,  with  an  average  potential  gradient  of  50,000  volts 
per  foot,  a  current  of  about  10,000  amperes. 

The  heating  effect  and  the  magnetic  effect  of  lightning 
strokes  also  point  to  the  passage  of  currents  of  some  thousand 
amperes. 

Assuming  then  the  average  potential  gradient  in  the  light- 
ning flash  as  50,000  volts  per  foot,  the  current  as  10,000  am- 
peres, a  lightning  flash  of  two  miles'  length  would  represent  a 
power  of  5  x  io9  K.  W.  Estimating  the  energy  of  the  discharge, 
as  approximated  from  the  photometric  consideration,  as  10,000 
K.  W.  seconds,  the  duration  of  the  discharge  would  be: 
io*/5  x  io9  =  2  x  io~6  sec.,  or  two-millionths  of  a  second. 

The  discharge  probably  is  oscillatory.  In  view  of  the 
high  resistance  of  the  discharge  path,  the  damping  effect  must 
be  very  great,  that  is,  a  very  large  part  or  nearly  all  the  energy 
is  expended  in  the  first  half-wave ;  that  is,  the  discharge  consists 
of  only  one  or  very  few  half- waves.  With  a  duration  of  the 
discharge  of  2  x  io"6  seconds,  assuming  two  half- waves  as 
an  average,  gives  500,000  cycles. 


LIGHTNING  AND  LIGHTNING  PROTECTION    269 

The  frequency  of  oscillation  of  the  lightning  flash  thus 
appears  to  be  of  the  magnitude  of  half  a  million  cycles. 

Since  the  velocity  of  propagation  of  electric  disturbances 
is  the  velocity  of  light,  or  188,000  miles  per  second,  the  wave 

188,000  3 
length  of  a  discharge  of  500,000  cycles  is =  —  miles, 

500,000  8 
or  about  2000  feet. 

A  wave  length  of  2000  feet  means  that  the  current  in  the 
discharge  flows  in  one  direction  for  1000  feet,  in  the  opposite 
direction,  that  is,  with  opposite  potential  gradient,  in  the  next 
thousand  feet,  etc.  That  is,  in  our  former  discussion,  the 
average  distance  through  which  the  potential  gradient  has  the 
same  direction,  or  the  distance  between  maximum  and  mini- 
mum, between  densest  and  lightest  parts  of  the  cloud  is  about 
1000  feet.  This  agrees  fairly  well  with  the  appearance  of  the 
clouds  to  the  eye,  and  it  also  agrees  in  magnitude  with  the  dis- 
tance over  which  the  wind  velocity  varies,  in  gusts,  as  shown 
by  Prof.  Langley  in  his  investigation  on  the  "internal  energy 
of  the  wind". 

It  appears  herefrom,  that  the  varying  wind  velocity  as 
measured  by  Prof.  Langley,  that  is,  the  gusty  character  of  the 
air  currents,  results  not  only  in  an  internal  mechanical  energy, 
which  the  bird  utilizes  for  soaring,  but  also  results  in  unequal 
moisture  distribution,  and  so,  when  condensation  occurs,  in  an 
"internal  electrostatic  energy"  of  the  thunder  cloud,  which  dis- 
charges as  lightning. 

With  an  average  length  of  the  half -wave  of  1000  feet, 
and  50,000  volts  per  foot  as  potential  gradient,  the  potential 
differences  in  the  clouds  would  be  of  the  magnitude  of  fifty 
million  volts.  These  are  values  which  appear  reasonable. 

Assuming  that  a  lightning  flash  drains  the  electric  energy 
of  the  cloud  within  a  radius  of  about  100  to.  200  feet  from  the 


270  GENERAL  LECTURES 

path  of  the  discharge,  this  affords  a  different  method  of  esti- 
mating the  magnitude  of  the  energy  of  the  lightning  flash: 
assuming  for  instance  saturated  air  at  40°  C  mixing  with  air 
at  o°C,  condensation  of  a  part  of  the  moisture  occurs  which 
can  easily  be  calculated.  Assuming  that  this  moisture  has 
conglomerated  to  rain  drops  of  .1"  to  .2"  diameter,  the  num- 
ber of  such  drops  in  a  space  of  two  miles'  length,  and  200  to 
400  feet  diameter,  can  be  calculated,  and  also  their  electro- 
static capacity.  With  a  wave  length  of  2000  feet,  and  a 
potential  gradient  of  50,000  volts  per  foot,  from  the  capacity 
follows  the  energy  of  the  electrostatic  charge,  which  dis- 
charges as  lightning  flash.  This  is  found  under  the  above 
assumption,  as  of  the  magnitude  of  10,000  K.  W.  seconds,  so 
agrees  with  the  results  derived  from  the  photometric  considera- 
tions. 

To  conclude  then,  as  approximate  values  of  magnitude 
of  the  electric  quantities  in  a  lightning  flash  may  be  estimated : 

Average  potential  gradient:  50,000  volts  per  foot  at  the 
moment  of  discharge. 

Average  potential  difference  between  different  points  of 
the  cloud :  50  million  volts. 

Average  current  in  the  discharge  10,000  amperes. 

I 

Average  duration  of  the  discharge sec. 

500.000 

Average  frequency  of  discharge :  500,000  cycles. 
Average  energy  of  the  discharge:  10,000  K.  W.  sec.,  or 
seven  million  foot  pounds. 

II.    LIGHTNING  IN  ELECTRIC  CIRCUITS. 
Of  greatest  importance  to  an  electrical  engineer  are  the 
high  potential  phenomena    produced    in    electric   circuits   by 
atmospheric  lightning  as  well  as  by  other  causes,  frequently 


LIGHTNING  AND  LIGHTNING  PROTECTION    271 

internal  to  (the  circuit,  which  give  the  same  or  similar  effects 
to  such  an  extent,  that  it  has  become  customary  when  dealing 
with  electric  circuits,  to  distinguish  between  external  or 
atmospheric  lightning,  and  internal  lightning,  as  caused  by 
electric  circuit  disturbances  or  defects,  such  as  sudden  changes 
of  load,  or  arcing  grounds,  etc. 

While  a  very  large  amount  of  data  on  high  potential 
phenomena  in  electric  circuits  has  accumulated,  the  possible 
variety  of  phenomena  is  so  great  that  an  intelligent  under- 
standing of  the  phenomena,  as  is  required  for  effective  pro- 
tection of  the  circuits,  is  feasible  only  by  a  theoretical  investi- 
gation of  the  high  potential  phenomena  which  may  be  expected 
in  electric  circuits,  and  a  comparison  thereof  with  the  observed 
effects. 

In  general,  the  high  potential  phenomena  possible  in 
electric  circuits  are  the  same  three  classes  of  phenomena  which 
can  occur  in  any  medium,  as  a  body  of  water,  which  is  the 
seat  of  energy. 

1.  Steady  electrostatic  stress,  that  is,  a  gradual  rise  of 
potential  of  the  total  circuit  against  ground,  until  a  discharge 
occurs  somewhere;  just  as  in  a  body  of  water,  as  a  river,  the 
pressure,  that  is,  the  water  level,  may  gradually  rise,  until  it 
breaks  through  the  embankment. 

2.  Impulses,  or  traveling  waves,  similar  to  the  ocean 
waves  rolling  over  the  surface  of  the  water. 

3.  Standing  waves,  or  oscillations  or  surges,  similar  to 
the  oscillation  of  a  tuning  fork,  or  a  violin  string. 

A  more  extended  discussion  on  the  three  forms  of  electric 
disturbances,  and  their  causes,  is  given  in  a  paper  read  before 
the  A.  I.  E.  E.* 

*A.  I.  E,  E.     Transact.  March,  1907:     "Lightning  Phenomena  in  Electric 
Circuits." 


272  GENERAL  LECTURES 

Steady  electrostatic  stress  obviously  can  occur  only 
where  the  circuit  is  very  well  insulated  from  the  ground,  but 
not  in  a  grounded  circuit,  or  a  leaky  circuit,  as  low  voltage 
circuits  usually  are,  and  such  static  stresses  can  be  eliminated 
by  a  permanent  leak,  that  is,  a  high  resistance  connection 
between  the  circuit  and  the  ground. 

As  sources  of  impulses  or  traveling  waves  only  two 
characteristic  phenomena  may  be  considered  here:  the  light- 
ning flash,  or  induction  by  the  clouds,  as  external,  and  the  arc- 
ing ground  as  internal  cause. 

Assuming  a  thunder  cloud  to  pass  over  the  line.  The 
ground  below  the  cloud  then  assumes  an  electrostatic  charge, 
corresponding  to  the  opposite  charge  of  the  cloud.  The  trans- 
mission line,  as  part  of  the  ground,  thus  also  assumes  a  static 
charge,  higher  than  that  of  the  ground,  since  it  projects  above 
it.  .  Any  equalization  of  the  potential  distribution  in  the  cloud 
by  a  lightning  flash,  as  discussed  in  the  preceding,  requires  a 
change  in  the  electrostatic  charge  of  the  line,  corresponding  to 
the  changed  potential  difference  between  ground  and  cloud 
above  the  ground,  and  the  static  charge  thus  set  free  on  the  line 
rushes  as  an  impulse  or  wave  along  the  line.  The  wave  shape  of 
such  impulses  induced  by  cloud  discharges  is  in  general  not  a 
smooth  sine  wave,  but  may  be  very  irregular:  during  the 
equalization  of  the  cloud  potential  by  the  lightning  flash,  the 
potential  difference  against  ground,  of  the  part  of  the  cloud 
above  the  electric  circuit,  may  vary  in  almost  any  conceivable 
manner,  thus  giving  rise  «to  very  different  wave  shapes  of  the 
impulses.  So  some  impulses  may  rise  very  rapidly,  with 
extremely  steep  wave  front,  and  slowly  die  down.  Others 
may  rise  slowly,  then  suddenly  fall  and  reverse,  or  a  series  of 
oscillations  may  occur  in  the  impulse,  etc.  If  the  lightning 
flash  is  parallel  with  the  line,  simultaneous  impulses  of  different 


LIGHTNING  AND  LIGHTNING  PROTECTION    273 

directions  may  be  produced,  corresponding  to  the  different 
directions  of  the  potential  gradient  in  the  different  parts  of  the 
lightning  flash,  and  these  waves,  of  different  directions, 
intensity  and  wave  length,  traveling  over  each  other,  then  pro- 
duce a  very  complex  system  of  phenomena.  So  for  instance, 
by  the  intereference  of  two  impulses  of  nearly  equal  wave 
length,  moving  in  opposite  directions,  a  high  voltage  point  may 
be  produced,  traveling  slowly  along  the  line,  and  visible  to  the 
eye  as  a  luminous  streak. 

The  frequencies  of  these  impulses  then  are  those  corres- 
ponding to  the  frequencies  of  cloud  discharge,  that  is,  of  the 
magnitude  of  hundred  thousands  of  cycles  per  second.  With  the 
velocity  of  light,  188,000  miles  per  second,  they  travel  along 
the  line,  until  they  gradually  fade  out  by  the  dissipation  of 
their  energy,  or  are  reflected  at  an  open  end  of  the  line,  or  at 
the  entrance  to  the  station  are  broken  up  by  partial  reflection, 
in  reactances,  and  interference  between  the  reflected  waves,  the 
incoming  waves  and  the  waves  passing  over  the  reactances, 
and  so  give  rise  to  systems  of  standing  waves  or  oscillations, 
similarly  as  an  ocean  wave  rolling  on  to  a  sloping  beach  breaks 
up  into  surf. 

Where  a  traveling  wave  is  reflected,  the  combination  of 
the  reflected  wave  and  the  incoming  wave  produces  a  standing 
wave  or  oscillation,  that  is,  a  wave  in  which  the  voltage  maxi- 
ma and  the  zero  points  or  nodes  have  fixed  positions  on  the 
line. 

By  superposition  of  the  wave  maxima  of  incoming  and 
reflected  wave,  the  standing  wave  rises  to  a  maximum  double 
that  of  the  traveling  wave.  Where  different  oscillations  or 
standing  waves  superimpose  upon  each  other,  their  maxima 
substract  at  some  places  and  add  at  others,  and  thus  again 
double  the  voltage,  that  is,  a  traveling  wave  or  impulse,  break- 


274  GENERAL  LECTURES 

ing  up  into  systems  of  oscillations  at  a  station,  doubles  and 
quadruples  the  potential;  so  that  a  traveling  wave  of  moder- 
ate potential  may  cause  dangerous  voltages  when  breaking  up 
into  oscillations,  just  as  in  the  ocean  surf,  the  waves  rise  to 
far  greater  heights  than  in  the  on-rolling  ocean  wave  before  it 
reaches  the  beach. 

If  we  consider  that  the  impulses  traveling  along  the  line 
are  not  sine  waves,  but  of  very  irregular  shape,  that  is,  can  be 
considered  as  consisting  of  a  fundamental  of  some  hundred 
thousand  cycles,  and  numerous  higher  harmonics  of  still 
greater  frequency,  and  each  of  the  components  when  breaking 
up  at  the  station  gives  rise  to  a  set  of  oscillations  at  every  inter- 
ference point,  that  is,  at  every  reactance,  the  complexity  of 
the  phenomenon  can  be  imagined. 

Since  the  equalization  of  cloud  potential  usually  occurs 
by  a  series  of  successive  discharges  in  short  intervals,  a  small 
fraction  of  a  second,  and  each  discharge  gives  rise  to  an 
impulse  in  the  line,  and  so  a  system  of  oscillations  at  the  sta- 
tion, whatever  protective  device  is  used,  must  restore  itself 
instantly  after  a  discharge,  so  as  to  receive  the  next  following 
discharge.  Any  device  depending  on  mechanical  motion  to 
restore  itself  after  a  discharge  to  operative  position,  therefore 
fails  to  protect,  when  a  series  of  discharges  follow  each  other 
in  very  rapid  succession,  as  discussed  above. 

Traveling  waves  very  similar  in  character  to  those  due  to 
induction  from  the  clouds,  but  frequently  of  far  greater 
volume,  sometimes  occur  in  an  electric  circuit  from  internal 
causes,  as  arcing  grounds,  or  spark  discharges. 

Let,  for  instance,  a  spark  occur  in  an  insulated  under- 
ground cable  system  between  one  of  the  conductors  and  the 
grounded  cable  armor,  through  a  weak  spot  in  the  insulation,  as 
a  faulty  joint  or  a  cable  bell.  Normally  a  potential  difference 


LIGHTNING  AND  LIGHTNING  PROTECTION    275 

exists  between  the  cable  conductor  and  the  ground,  equal  to  the 
Y  potential  of  the  system,  and  so  an  electrostatic  charge  on  the 
conductor  corresponding  thereto.  A  spark  passing  between 
conductor  and  ground,  connects  it  to  ground,  and  the  charge 
of  the  conductor  so  passes  over  the  spark  as  arc  to  ground. 
As  soon,  however,  as  the  conductor  is  discharged  and  at 
ground  potential,  the  arc  between  conductor  and  ground 
ceases,  since  there  is  no  voltage  left  to  maintain  it,  and  so  the 
conductor  disconnects  from  ground.  The  conductor  then 
charges  itself  again  to  its  normal  Y  potential  and  during  the 
in-rush  of  the  charge,  momentarily  the  potential  builds  up  to 
double  voltage.  Thereby  a  spark  again  passes  between  con- 
ductor and  ground,  discharges  it  again,  opens  after  discharge, 
again  causes  a  spark  to  pass,  etc.  So  a  series  of  successive 
sparks  occur  between  conductor  and  ground,  discharging  the 
conductor  by  currents  which  momentarily  rise  to  very  high 
values,  the  discharge  current  of  the  capacity  of  the  conductor 
against  ground,  over  a  path  of  practically  no  resistance.  Each 
spark  discharge  sends  out  an  impulse  or  traveling  wave,  and 
thus  a  spark  discharge  between  conductor  and  cable  armor,  or 
in  the  same  manner  an  arcing  ground  on  an  overhead  transmis- 
sion line,  as  is  for  instance  caused  by  a  broken  insulator,  pro- 
duces a  continuous  series  of  impulses  or  traveling  waves,  which 
follow  each  other  with  the  rapidity  of  charge  and  discharge  of 
the  cable  or  the  line,  that  is,  many  thousands  per  second,  and  so 
give  what  has  been  called  a  recurrent  surge.  In  a  long  distance 
transmission  line,  the  frequency  of  the  recurrent  surge  usually 
is  somewhat  lower  than  in  an  underground  cable  system,  but 
is  still  thousands  of  impulses  per  second. 

The  frequency  of  oscillations  occurring  in  electric  cir- 
cuits varies  over  an  enormous  range:  from  low  frequencies, 
very  little  above  alternator  frequency,  up  to  hundreds  of  mil- 


276  GENERAL  LECTURES 

lions  of  cycles  per  second ;  and  the  effect  of  the  oscillations  in 
the  system  therefore  varies  accordingly:  from  the  relatively 
harmless  static  displays;  brush  discharges,  streamers,  sparks, 
etc.,  of  extremely  high  frequencies,  down  to  the  disastrous 
high  power  low  frequency  short  circuit  oscillations,  in  which 
even  in  10,000  volt  system*,  currents  of  many  thousands  of 
amperes  may  surge,  which  voltages  approaching  100,000,  and 
with  which  no  protective  device  can  cope,  which  does  not  have 
unlimited  discharge  capacity,  that  is,  contains  no  resistance 
whatever  in  the  discharge  path. 

III.     LIGHTNING  PROTECTION  OF  ELECTRIC 
CIRCUITS. 

From  the  preceding  considerations  it  follows  that  the 
problem  of  protecting  electric  circuits  from  lightning  is  two- 
fold: 

1.  To  guard  against  high  potential  disturbances  enter- 
ing the  circuit  from  the  outside  or  originating  in  the  circuit. 

2.  To  discharge  harmlessly  to  ground,  whatever  high 
potential  phenomena  may  appear  in  the  circuit. 

From  atmospheric  electric  disturbances,  complete  protec- 
tion can  be  secured  by  putting  the  circuit  under  ground,  or, 
where  this  is  not  feasible,  to  put  the  ground  over  the  electric 
circuit.  This  means  the  use  of  grounded  overhead  wires.  The 
overhead  ground  wires  so  protect  the  circuit  the  more  com- 
pletely, the  more  they  realize  a  complete  shield  interposed 
between  line  and  sky.  While  complete  protection  thus  would 
require  a  system  or  network  of  grounded  conductors  above, 
beside,  and  also  below  the  transmission  line,  very  good  protec- 
tion in  most  cases  is  secured  by  a  single  ground  wire  of  good 
conductivity,  installed  well  above  the  line;  and  in  no  place  of 


LIGHTNING  AND  LIGHTNING  PROTECTION    277 

electric  transmission  systems  can  money  be  more  efficiently 
spent,  than  in  securing  good  overhead  ground  wire  protection. 

To  guard  against  the  appearance  of  internal  lightning 
requires  constant  watchfulness  in  the  design,  construction  and 
operation  of  the  system,  to  avoid  all  conditions  which  may  lead 
to  the  formation  of  oscillating  arcs.  Thus  poor  contacts,  loose 
joints,  masses  of  insulated  metal  near  high  potential  con- 
ductors, etc.,  should  be  carefully  avoided. 

The  disturbances  which  have  to  be  taken  care  of  by  the 
lightning  arresters  proper,  are  steady  accumulation  of  static 
pressure;  impulses  or  traveling  waves;  oscillations  or 
surges ;  occurring  singly  or  in  groups,  and  of  frequencies  vary- 
ing between  many  millions  of  cycles  and  ordinary  machine 
frequencies;  and  recurrent  surges,  that  is,  impulses  and  oscil- 
lations, usually  of  high  frequency,  following  each  other  in  very 
rapid  succession,  usually  thousands  per  second. 

It  is  necessary  that  the  discharge  over  the  lightning 
arrester  should  occur  with  the  least  possible  disturbance  to  the 
system,  that  is,  the  discharge  current  should  be  as  small  as  per- 
missible without  causing  a  voltage  rise  due  to  the  resistance  of 
the  discharge  path.  At  the  same  time,  the  protective  devices 
must  be  able  to  discharge  practically  unlimited  currents,  that 
is,  currents  of  the  magnitude  of  the  momentary  short  circuit 
current  of  the  system.  This  obviously  requires  that  the  pro- 
tective devices  should  have  no  appreciable  resistance  in  the 
discharge  path.  Any  lightning  arrester  containing  series 
resistance  obviously  fails  to  protect  as  soon  as  the  discharge 
current  is  so  large  that  the  ohmic  drop  across  the  resistance 
becomes  serious,  and  the  maximum  discharge  current  which 
may  occur,  is  the  short  circuit  current  of  the  system,  that  is, 
extremely  large. 


278  GENERAL  LECTURES 

Three  types  of  protective  devices  are  at  present  available. 

1.  The  circuit  is  connected  to  ground  by  a  single  spark 
gap  set  for  a  voltage  exceeding  the  normal  operating  voltage 
by  a  safe  margin :  the  so-called  horn  gap,  or  goat  horn  light- 
ning arrester.    As  soon  as  the  voltage  rises  beyond  the  value 
for  which  the  spark  gap  is  set,  it  discharges,  and  the  system 
is  short  circuited  to  ground,  until  the  arc  rises  and  gradually 
blows  itself  out.    As  this  requires  an  appreciable  time,  motors 
and  converters  have  usually  dropped  out  of  step,  and  the  gen- 
erators have  broken  synchronism,  that  is,  the  system  is  shut 
down  and  has  to  be  started  up  again.   This  type  of  protection 
therefore  is  not  particularly  favored  in  systems  which  require 
reasonable  continuity  of  service,  but  if  used,  it  is  considered 
rather  as  an  emergency  device  in  addition  to  other  arresters  and 
is  then  adjusted  for  much  higher  discharge  vokage.  A  reduction 
of  the  current  over  the  horn  gap  by  series  resistance  is  not  per- 
missible, since  it  correspondingly  reduces  the  protective  value, 
as  explained  above,  and  the  arrester  ceases  to  protect  against 
a  high  power  surge.     While  such  surges  are  relatively  infre- 
quent, their  destructiveness  is    such    that    protection    against 
them  is  especially  needed.    Fuses  in  series  with  the  horn  gap, 
if  they  open  slowly,  would  still  shut  down  the  system,  and 
if  opening  very  rapidly,  the  shock  of  the  explosive  opening  of 
the  fuse  on  the  short  circuit  current  of  the   system   may  be 
disastrous.    Obviously,  the  use  of  series  fuses  require  a  multi- 
plicity of  spark  gaps  to  give  continuity  of  protection. 

2.  The  type  of  lightning  arrester  now  almost  universal- 
ly used  is  the  multi-gap   arrester,    which    short    circuits    the 
system  for  one-half  wave  only.    It  consists  of  a  large  number 
of  spark  gaps  between  metal  cylinders,  in  series  with  each  other. 
As  now  designed,  different  sections  of  the  gaps  are  shunted 
with  different  resistances,  for  the  purpose  of  affording  equal 


LIGHTNING  AND  LIGHTNING  PROTECTION  279 

protection  against  all  frequencies,  and  adjusting  automatically 
the  resistance  of  the  discharge  path  to  the  volume  of  the  dis- 
charge, as  for  instance,  discharge  slow  accumulations  of  poten- 
tial over  a  very  high  resistance,  short  circuit  surges  over  a 
path  of  zero  resistance,  and  thus  pass  a  discharge  with 
the  minimum  shock  on  the  system.  The  operation  of  the 
multi-gap — which  by  the  way  is  suitable  only  for  alternating 
current  systems — depends  on  the  non-arcing  character  of  cer- 
tain metals.  Metals  of  low  boiling  point,  as  mercury  or  zinc, 
cannot  maintain  an  alternating  current  arc,  but  the  arc  goes 
out  when  at  the  end  of  the  half  wave,  the  current  falls  to  zero, 
and  a  very  much  higher  voltage  is  required  to  again  start  an 
arc  for  the  next  half-wave.*  Alloys  of  such  metals,  usually 
zinc,  with  metals  of  high  melting  point,  as  copper,  are  therefore 
used  as  (terminals  in  the  multi-gap  arrester. 

A  discharge  over  the  multi-gap  arrester  short  circuits  the 
system  for  the  rest  of  the  half- wave  during  which  the  discharge 
occurs.  At  the  end  of  the  half- wave,  the  current  falls  to  zero, 
and  the  reverse  current  cannot  start,  that  is,  the  circuit  of  the 
arrester  is  opened. 

A  short  circuit  on  the  system,  for  a  fraction  of  a  half- 
wave,  does  not  interfere  with  the  operation  of  synchronous 
apparatus,  that  is,  the  operation  of  the  system  is  not  affected 
by  a  discharge  over  the  multi-gap  arrester. 

In  a  large  system,  the  short  circuit  current  is  very  consid- 
erable ;  its  power,  and  thus  the  heating  effect  produced  by  it,  is 
enormous.  The  energy,  and  thus  the  heat  produced  by  the  short 
circuit  current  during  the  fraction  of  the  half -wave,  which  the 
discharge  over  the  multi-gap  arrester  lasts,  is  moderate,  due  to 
its  very  short  duration,  and  can  easily  be  absorbed  and  radiated 

See  paper  A.  I.  E.  E.     Transact.  1906,  p.  789.      "Transformation  of  Electric 
Power  into  Light. ' ' 


280  GENERAL  LECTURES 

by  the  arrester;  so  that  even  if  lightning  discharges  rapidly 
follow  each  other  for  some  time,  they  can  be  taken  care  of  by 
the  arrester  with  moderate  temperature  rise :  assuming  a  vicious 
thunder  storm,  in  which  lightning  flashes  succeed  each  other 
practically  continuously,  several  per  second.  Each  discharge 
causes  a  short  circuit  over  the  lightning  arrester,  varying  in 
duration  from  nearly  a  half-wave — if  the  discharge  occurs  at 
the  beginning  of  a  half- wave — to  practically  nothing — if  the 
discharge  takes  place  near  the  end  of  a  half-wave — that  is,  in 
average,  for  one-half  of  one-half  wave,  or  1/240  sec.,  in  a  60 
cycle  system.  Therefore  from  two  to  three  lightning  dis- 
charges per  second  would  still  short  circuit  the  system  over  the 
multi-gap  arrester  only  for  i  %  of  the  total  time,  and  the  heat- 
ing effect,  caused  by  a  short  circuit  during  i  %  of  the  time,  can 
be  taken  care  of  by  the  arrester  for  a  considerable  period. 

Let  us  see,  however,  what  happens  to  the  multi-gap  light- 
ning arrester  in  case  of  the  appearance  of  a  recurrent  surge, 
as  an  arcing  ground,  that  is,  discharges  following  each  other 
in  rapid  succession,  thousands  per  second.  The  first  discharge, 
passing  over  the  lightning  arrester,  short  circuits  the  system 
for  the  rest  of  the  half-wave,  and  at  the  end  of  the  half-wave, 
the  arrester  functionates  properly,  that  is,  opens  the  circuit.  At 
the  next  moment,  however,  at  the  beginning  of  the  next  half- 
wave,  the  next  oscillation  of  the  recurrent  surge  again  dis- 
charges over  the  arrester,  and  thus  again  short  circuits.  That 
is,  with  a  recurrent  surge,  the  multi-gap  arrester  at  the  end 
of  every  half-wave  opens  the  circuit,  at  the  beginning  of  the 
next  half -wave,  the  next  oscillation  of  the  recurrent  surge 
short  circuits  again.  As  far  as  the  effect  on  the  operation  of 
the  system,  and  the  heating  of  the  arrester  is  concerned,  a 
recurrent  surge  causes  a  permanent  short  circuit  on  the 
system,  except  that  at  the  beginning  of  every  half-wave,  for  a 


LIGHTNING  AND  LIGHTNING  PROTECTION    281 

short  period,  the  circuit  is  opened  and  free  for  the  appearance 
of  disruptive  voltages  elsewhere,  and  so  apparently,  simul- 
taneous with  the  short  circuit,  destructive  high  potentials  may 
appear  in  the  system.  The  heating  effect  of  the  short  circuit 
current,  which  occurs  at  every  half -wave,  rapidly  destroys  the 
arrester.  In  such  cases,  to  save  the  arrester,  it  has  been  cus- 
tomary to  insert  a  series  of  auxiliary  gaps,  which  are  thrown 
in  by  the  blowing  of  a  fuse  shunting  them,  and  raise  the  dis- 
charge voltage  of  the  arrester  so  that  the  recurrent  surge  does 
not  pass  over  it.  It  is  obvious,  that  in  this  case  the  arrester 
ceases  to  protect  the  system  against  the  recurrent  surge:  but 
if  left  in  circuit,  the  destruction  of  the  arrester  would  put  it 
out  of  operation  anyway. 

It  is  obvious  now,  that  no  lightning  arrester,  which  func- 
tionates by  short  circuiting  the  system  for  the  rest  of  the  half- 
wave,  during  which  a  discharge  occurs,  can  take  care  of  and 
protect  against  a  recurrent  surge,  since  the  proper  functionating 
of  the  arrester,  with  a  recurrent  surge,  represents  a  permanent 
short  circuit  on  the  system  over  the  arrester,  and  so  a  destruc- 
tion of  the  arrester,  no  matter  whose  make  it  may  have  been, 
and  a  shutdown  of  the  system. 

3.  To  take  care  of  a  recurrent  surge,  a  protective  device 
would  thus  be  required,  which  does  not  short  circuit  the  system 
even  for  one  half- wave,  but  which  never  allows  the 
normal  voltage  of  the  system  to  pass  a  current  over  the 
arrester,  but  acts  as  a  short  circuit  for  any  excess  voltage  above 
the  normal  voltage.  The  possibility  of  such  a  device  we  can 
understand  by  considering  the  effect,  which  in  a  direct  current 
circuit  a  storage  battery  would  have,  when  shunted  between  the 
circuit  and  the  ground.  Assuming  for  instance  in  a  500  volt 
trolley  circuit,  a  500  volt  storage  battery  of  very  high  capacity, 
that  is,  negligible  internal  resistance,  permanently  connected 


282  GENERAL  LECTURES 

between  line  and  ground.  With  the  normal  line  potential  of 
500  volts,  no  current  would  pass  over  the  battery  to  ground, 
except  the  very  small  current  required  to  maintain  the  battery 
charged.  No  rise  of  voltage,  however,  could  occur  in  the 
system  by  lightning  or  any  other  cause,  since  any  voltage  above 
500  volts,  the  counter  e.  m.  f.  of  the  battery,  would  be  short 
circuited  to  ground  through  the  battery,  and  such  a  battery 
would  thus  give  perfect  protection  against  any  high  voltage  dis- 
turbances in  the  system.  In  case  of  a  recurrent  surge,  the  cur- 
rent discharging  over  the  battery  would  be  the  short  circuit 
current  of  the  excess  voltage,  that  is,  the  surge  potential,  and 
the  heating  effect  of  this  current  is  negligible,  since  high 
potential  high  frequency  phenomena  are  of  limited  power  and 
especially  of  limited  current,  as  condenser  discharges. 

A  storage  battery  obviously  is  not  suitable  for  alternating 
current  and  would  not  be  practical  in  any  case,  as  it  requires 
a  cell  for  every  two  volts.  The  same  effect,  however,  is  pro- 
duced at  a  much  higher  voltage,  in  an  alternating  current  cir- 
cuit, by  the  aluminum  cell.  If  such  a  cell,  consisting  of  two 
aluminum  plates  in  certain  electrolytes,  is  exposed  to  an  alter- 
nating voltage,  a  film  forms  on  the  aluminum  plates,  which 
holds  back  the  impressed  voltage,  that  is,  acts  like  a  counter 
e.  m.  f.  equal  to  the  impressed  e.  m.  f.,  so  that  practically  no 
current  passes  through  the  cell,  or  only  the  small  current 
required  to  maintain  the  film,  of  a  magnitude  of  about  .01 
amperes  per  square  inch  plate  surface,  while  for  any  sudden 
rise  of  voltage  the  cell  acts  as  a  short  circuit  for  the  excess 
voltage.  Over  the  storage  battery,  the  aluminum  cell  has  the 
advantage  of  higher  voltage:  a  single  cell  can  take  care  of 
300  to  400  volts  and  even  more,  and  also  that  it  does  not  have 
a  fixed  counter  e.  m.  f.,  but  a  counter  e.  m.  f.,  which  adjusts 
itself  to  equality  with  the  impressed  voltage,  at  any  value  up 


LIGHTNING  AND  LIGHTNING  PROTECTION    283 

to  about  600  volts  per  cell.  Assuming  for  instance  an  alumi- 
num cell  connected  across  an  alternating  e.  m.  f.  of  300  volts. 
With  the  film  formed,  a  negligable  current  passes  through 
the  cell,  for  instance,  1/4  of  an  ampere,  maintaining  the 
integrity  of  the  film.  If  now  the  voltage  is  suddenly  raised 
to  330  volts,  in  the  first  moment  the  cell  acts  as  a  short  circuit 
of  the  excess  voltage,  in  this  case,  30  volts,  and  for  an  instant 
a  very  large  current,  possibly  hundreds  of  ampers  if  the  supply 
source  is  capable  of  giving  such  a  current,  rushes  through 
the  cell.  This  current  very  rapidly  decreases,  by  the  film  of 
the  aluminum  plates  forming  for  higher  voltage,  so  that  in  a 
few  seconds  the  current  is  already  small,  and  in  a  few  minutes 
the  normal  current  of  1-4  ampere  again  passes,  but  now  at  330 
volts  impressed,  and  the  film  has  formed  to  a  counter  e.  m.  f. 
equal  to  this  higher  voltage,  probably  has  thickened.  If  now 
we  again  lower  the  voltage  suddenly  to  300,  in  the  first  moment 
the  current  in  the  cell  practically  disappears,  and  then  graually 
rises  again,  and  after  a  few  minutes  is  again  normal  at  1/4 
ampere,  that  is,  the  film  has  built  down  again  to  300  volts.  In 
this  manner  the  aluminum  cell  adjusts  its  counter  e.  m.  f.  to 
changes  of  impressed  voltage,  by  the  film  building  up  or  build- 
ing down.  This  adjustment,  for  moderate  voltage  variation, 
as  may  be  expected  when  varying  the  generator  voltage  of  the 
system,  is  quite  rapid,  most  of  the  change  occurring  within 
less  than  a  second,  but  is  still  extremely  slow  compared  with 
the  rapidity  of  lightning  phenomena,  and  for  lightning 
phenomena  .the  aluminum  cell  therefore  acts  as  a  short  circuit  of 
the  excess  voltage  above  the  normal  machine  voltage.  Thus  the 
recurrent  surge,  with  a  system  of  aluminum  cells  in  series  with 
each  other  connected  directly  across  the  circuit,  cannot  produce 
any  rise  of  voltage,  but  the  excess  voltage  over  the  normal,  or 
the  surge  potential,  is  short  circuited  through  the  aluminum 


284  GENERAL  LECTURES 

cell,  so  causing  a  small  increase  of  the  current  in  the  cells,  by 
the  superposition  of  the  high  frequency  surge  current  over  the 
normal  leakage  current  of  the  cell,  but  no  rise  of  voltage.  Since 
the  recurrent  oscillations  are  intermittent,  obviously  the  film 
of  the  aluminum  cells  cannot  build  up  to  their  voltage,  but 
remains  corresponding  to  the  machine  voltage,  that  is,  the 
aluminum  cell  can  permanently  discharge  a  recurrent  surge 
without  any  short  circuit  of  the  main  voltage,  or  any  disturb- 
ance on  the  system. 


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